You touch it with the tip of your tongue, where a quarter of your 10,000 taste buds best savour sweets. It melts slowly in your mouth, at a temperature just below your body's own, and as you swallow, it passes through your system like a rush of amphetamine. A chemical called phenylethylamine is released in your brain, along with serotonin, that feel-good neurotransmitter familiar to the world since the advent of Prozac. And along with this strange contradiction - a rush of calm - come the results of a magical ingredient: theobromine, a substance not unlike caffeine whose name means 'food of the gods'. You feel, within minutes, as though you have taken a mild and curious drug, the effects of which are neurologically identical to the act of falling in love.Yet all you have done is taken a bite of chocolate.

Ever since chocolate was introduced to the Western world 500 years ago, it has been valued for its medicinal, and feared for its decadent, qualities. Hernan Cortes, who first tasted it in the sixteenth century at the palace of the Aztec emperor Moctezuma, wrote to his King, Carlos V, that: 'Chocolatl is the divine drink that builds up resistance and fights fatigue. 'A hundred years later, Hans Sloane, the distinguished physician who succeeded Newton as president of the Royal Society, introduced chocolate to London, where it became a fashionable drink. Sloane invested a good deal of money in importing cocoa powder from the West Indies, which he said should be mixed with hot milk, like another remedy popular at the time, laudanum. Chocolate and drugs were imagined hand in hand.

Chocolate was thought to cure TB, gout, ulcers. The writer Mme de Sévigné, an early chocoholic, worried that she must kick the habit: 'All those who spoke well of it now tell me bad things about it, cursing it, and accusing it of all the ills that exist,' she wrote to her daughter in 1671. 'It is the cause of vapours and palpitations. It is pleasant for a while, but then suddenly lights up a continuous fever which leads to death.'

The next century saw the Marquis de Sade, imprisoned for his sexual practices, demanding that a cake be brought to him in jail: 'I want it to be chocolate,' he said, 'and black inside from chocolate as the devil's ass is black from smoke. And the icing is to be the same.' Once he spiked chocolate pastilles with an aphrodisiac at a ball, and is reported to have 'enjoyed the favours of his sister-in-law'.

In 1908, long after Joseph Fry had invented the first solid chocolate bar, the Swiss confectioner, Theodor Tobler, was inspired to make the Toblerone when he saw a show at the Folies Bergères in Paris, at the end of which a chorus of barely dressed women formed a human pyramid. No matter what its curative properties, chocolate has long been thought of as an aphrodisiac, a prelude to sex or a replacement for it and to blame for all manner of psychosexual ills. Even now, people have an exceptionally personal and emotional attachment to chocolate. They are loyal to their favourites: they will come down strongly on the side of Aero or Wispa; devotees of Twix, Mars bar or Fruit and Nut rarely stray. Embark on a discussion of commercial chocolate bars, and before long the person you are speaking to will remember, with sudden hurt and fury, the day 12 years ago when they changed the name of the Marathon bar to Snickers.

Part of the reason for this deeply ingrained nostalgia is that most of the best-selling brands were invented years ago, many of them in the Thirties. According to the late chocoholic Roald Dahl, 'the chocolate revolution had not yet begun' when he was growing up in England in the Twenties. True, there was Cadbury's Dairy Milk (1905) and Fry's Turkish Delight (1914), but the Twenties were the beginning of a great period of innovation. The Flake, brought out in 1920, was 'a milestone', according to Dahl, since 'it was the first time any manufacturer had seriously played with chocolate in their inventing rooms'. Fruit and Nut came in 1928, , the Crunchie a year later. The Aero began in 1935 and 1937 was a bumper year: Tiffin, KitKat, Rolos and Smarties.

The Mars bar was the brainchild of a foreigner, who launched his product in England in 1932. Forrest Mars was the son of Frank Mars, the American who had patented the Milky Way. Frank handed over his business to his son, who came to Slough armed with the recipe for Milky Way and set about inventing the first 'chew bar', something Cadbury's had attempted, without much initial success, with the early Curly-Wurly, since it was very hard to get the chocolate coating to stick to the caramel. Forrest Mars managed it, and his Mars Bar was a sensation. He later moved back to America and invented, among other things, Maltesers, the Snickers bar and M&Ms - which he was accused of stealing from Rowntree's Smarties, though he claims to have thought of them long before, when he saw soldiers during the Spanish Civil War eating sugar-coated chocolate pellets.

Now, however, the chocolate industry - which in Britain grosses £3.8 billion a year - is not run by inventors, or by philanthropists such as Cadbury or by apothecaries like Fry. It's run by marketing experts. Yorkies are now 'not for girls'; a Flake is phallic; Galaxy is sensuous. While researching this article, I was told that the new Snickers Cruncher was aimed specifically at 'the 25-35 year-old woman', because it had fewer calories than a normal Snickers, and yet it was advertised, bafflingly, with a pair of boxing gloves. Well over £100 million a year is spent advertising chocolate in this country; as a result, chocolate is never actually presented as chocolate. It's shown to be all about sex,or lifestyle,or toughness.

It's true that some chocolate suffers from an image problem: what, for example, to do about Revels? Are they doomed to appeal only to the indecisive? How about the rather matronly Turkish Delight? Who ever craves a Munchie? Are Creme Eggs only designed for vicarsat Easter? But more curious is the question of why, when all the best-selling brands are three-quarters of a century old, anyone would want to produce a new one.Honeycomb Aero? Galaxy Liaison? Limited Edition KitKat? Whose idea were these things,and why?

In order to find out, I went to the oldest chocolate factory in Switzerland, Cailler-Nestlé, a picturesque place embedded in the Alps almost surreally in the manner of a Milka wrapper. I am given a thin paper cap and a lab coat, and am led through rooms full of delicious gloop: the huge conches where the chocolate is stirred, the production lines where the sweets are moulded, the row of robots that fill the boxes.

In the Nestlé research centre, not far from the factory, chocolate is designed, tested and tasted. There are floors full of chemistry labs,and a set of sophisticated tasting booths, each with its own atmosphere to enable tasters to take in the smell of the chocolate,and equipped with trick lighting so that no one can tell the difference between dark and milk chocolate.

It is here that I meet Arthur Day, the man behind Nestlé-Rowntree's first new chocolate bar in five years, Nestlé Double Cream. Arthur works in Nestlé's 'innovation department', and has as his motto a saying of silicon guru Alan Kay: 'The best way to predict the future is to invent it.' Arthur's job is partly to research sociological change. 'Since 11 September,' he tells me, 'people are living for today. They're indulging themselves more.' What better for this newly hedonistic crew than a chocolate bar made with cream, and with sweet beans imported from Ecuador?

While conducting market research, Cadbury's discovered - by studying the success of Heinz Baked Beans - that the British preferred bland tastes. Almost instantly, they brought out the Double Decker. To this day many of our commercial chocolate bars fall short of EU regulations, which specify at least 35 per cent cocoa solids in dark chocolate, at least 25 per cent in milk chocolate.

Arthur says our tastes have changed since then. He compares our gradual chocolate snobbery to our increased knowledge of wine. In the beginning, he says, 'there was Blue Nun, which is the equivalent of Dairy Milk. Then we started drinking French wine, which you could say is when we became aware of luxury chocolates like Lindt. Now we're branching out, into Australian wine, for example, and yet there's no mass-market chocolate equivalent. We saw a need for a mainstream milk chocolate offering.' Now, Arthur explains, consumers put more of a premium on ingredients. They want 'realness', he says, natural products, and classy forms of indulgence. In other words, Nestlé Double Cream is a commercial response to a significant change: an increased awareness of health, and hence of 'real' chocolate (that is, in the case of dark chocolate, 70 per cent cocoa or more) which began in England in about 1983, when Chantal Coady opened her shop Rococo Chocolate in London's King's Road, and was consolidated in 1990 with the founding of the Chocolate Society.

Russell Hind, the manager there, agrees with Arthur Day that people's tastes are changing. 'They're learning what real chocolate tastes like,' he says. 'I see people's reactions every day - they've never had anything like it before. Chocolate is good for you, as long as you're eating the right stuff.' The Chocolate Society's new organic bar just won the organic bar of the year award from the Soil Association. 'There was a demand for it,' Hind says, 'people kept asking about organic, and were more conscious of Fairtrade.'

Since Hind has brought the subject up, I ask him about slavery, for which the cocoa farms in the Ivory Coast - the world's largest single producer of cocoa - are infamous. He says that the Chocolate Society knows exactly which plantations its cocoa comes from - they have a lot of their own, or share them with Valrhona, a superior luxury brand. 'I can't categorically say it doesn't exist, but it's more likely with commercial chocolate,where they don't really care where the beans come from.'

But if the Chocolate Society's political line is too flimsy for your conscience, go to Green and Black's, makers of the very first organic chocolate bar in 1991. The company was founded by Craig Sams, who had started up the organic food company Whole Earth, and his wife, writer Josephine Fairley. Sams is now chairman of the Soil Association. As world cocoa prices plummeted Green and Black's began to trade directly with cocoa farmers in Belize. They added a premium for keeping the produce organic, rather than planting hybrid trees, as they had been encouraged to do by an unnamed large chocolate corporation. They use no pesticides, and their Maya Gold bar, which was the first product ever to be awarded the Fairtrade mark, is to my mind the most delicious on the market.

But then, perhaps the politics of chocolate will be the last thing on your mind in those first few moments of tasting it, that rush of love. As Brillat-Savarin put it in his hymn to the stuff: 'Happy chocolate, which having circled the globe through women's smiles, finds its death at their lips in a delectable, melting kiss.'

· Dark chocolate and cherry 'crème brûlée'

Crème brûlée is one of the all-time greats in the pudding repertoire. Sometimes, when faced with a choice of a chocolate pudding or a crème brûlée, it can be an agonising decision. Here, there is no contest! This version, devised to incorporate chocolate, is also so much easier to make than a classic crème brûlée.

Serves 8

500g very dark cherries,preferably Morello if available, stoned
finely grated zest of 1 lemon
500ml crème fraiche
500g real dark chocolate,finely chopped
about 250g white caster sugar
125g unsalted butter,diced

Put the stoned cherries into the bottom of eight small ramekin dishes and sprinkle with the lemon zest. To make a ganache, chop the chocolate into squares and use a food processor to make it into a fine powder. Make sure there are no big lumps. Put chocolate into a heatproof bowl. Scald the creme fraiche in a pan - allow it to boil and rise up and pour a tablespoon on to the chocolate and mix well. Keep adding the crème fraiche, a spoonful at a time, mixing thoroughly. When all the crème fraiche has been mixed in, add the butter. The mixture should still be warm enough to melt it, although it will take a few minutes to beat it in, so there are no lumps left. Pour the ganache over the cherries and chill for at least two hours. Heat the sugar and six tablespoons of water in a pan until you have a bubbling caramel mixture. Let settle briefly, then spoon carefully over the chilled pots of cherry and ganache, it should set instantly as soon as it comes into contact with the cold mixture. These are best served immediately.

· To order a copy of Real Chocolate by Chantal Coady, for £12.99 plus p&p (rrp £14.99), call the Observer book service on 0870 066 7989. Published by Quadrille on 7 March.