What does a chef do once he's conquered the globe? When it's Alain Ducasse, become a chocolatier...

Walk out of Bastille station in Paris and take a turn down the Rue de la Roquette. Go past the tattoo parlour and the Korean barbecue and the hotels with the paint peeling from their fronts and you soon reach number 40, a tall, thin building set back from the road.

Google maps might assure us that it's a Renault garage but, as you approach its entrance, crossing the cobbled courtyard, it is apparent that whatever is sold inside, it ain't exhausts. With its glass front and hotel foliage, it might be a jewellers or a boutique opened as the first step in some grand plan to gentrify the area. But then you see the hessian sacks, piled one on top of each other like Tetris blocks. There is that smell, too: earthy, hot, almost like coffee. And you notice that in the glass cabinets, under those low-hanging lanterns, are not diamonds, sapphires and rubies but pralines, truffles and ganache. This is La Manufacture du Chocolat, a very unusual chocolate factory.

There are several things that mark La Manufacture out from its many competitors. Not merely its unusually pretty staff – to enter is to be hit by a tidal wave from the top of the gene pool – nor the fact that the chocolate is described in terms more readily associated with wine – "cru" and "domains" loom large – or even that it sells a 100 per cent cacao bar (a taste of which makes you feel a tiny bit dizzy). No, the most surprising thing here is the words, spelt out in foot-high letters, above the door – Alain Ducasse.

Now Ducasse is known for many things. He was the first man to have three restaurants in three cities with three Michelin stars each, for a start, and has a culinary empire which straddles North America, Europe, the Middle East and Asia and includes the restaurant at The Dorchester Hotel, but as a chocolatier he's an unknown.

So when I ask from where the idea for a chocolate factory came, his response is surprising. "I've always been thinking about chocolate," he says. Ever since he worked for pastry chef Gaston Lenôtre in the mid-Seventies, in fact. "I was captivated by its taste, richness and [the] subtlety of cocoa." So why, oh why, wait 30-odd years to make your own? "I didn't want to become a chocolatier among others, buying ready-to-use couverture. I wanted to take the same approach I follow in my cuisine: putting the product first, revealing the authentic taste of the products."

Confirmation of that, if confirmation is needed, comes with a quick shufti around the shop with his chief chocolatier. Nicholas Berger is a tall man with a welcoming Gallic face, who would, if he were to go on Mastermind, undoubtedly choose La Manufacture's chocolate as his specialist subject.

Point at that coconut praline or this passion-fruit ganache and he can discourse on it until you feel like you're personally acquainted with the tree which bore the cocoa bean. But then that's the whole point of the operation – La Manufacture is the only bean-to-bar chocolate-maker in Paris. "We do things the traditional way," says Berger. "I choose the beans first – from Vietnam, Peru, Trinidad, Madagascar – then I think what type of chocolate I want to make from them."

Berger's approach may sound simple but, in fact, it proved wickedly difficult. Chocolate is now a product of daily consumption for most of us, and the machines used to make it are commensurately massive. Even the smallest modern equipment has a capacity measured in tonnes; Berger, however, wanted to work in 100kg multiples. So, like many foodie start-ups before them, they scoured the second-hand market.

"I travelled all over Europe to find the machines we needed – and many of them I had to repurpose," he says. Thus, the cocoa-bean roaster was once used for coffee; the sorter, for Jordanian almonds; and the Fryma Mill to grind mustard seeds. Berger can now, perhaps unsurprisingly, strip down and rebuild each of his machines without breaking a sweat.

That may make him sound like a technician, but both he and Ducasse have approached chocolate-making like artisans, forging and recasting the rules as they have gone. Not for them the straight jacket of only using 75 per cent cacao, a quantity beloved of other chocolatiers. There is no ideal, says Ducasse. "The proportion of ingredients is important but the final result is also a matter of how you put them together. Equilibrium is key."

When you taste the chocolates, the fruity ganaches, the bonbons, the 44 different types of bar, you begin to see what he means. They are a United Nations of flavour. Take the 75 per cent cocoa bar from the Dominican Republic, for instance. It seems to contain a shopping basket of flavour: one minute hints of tea, the next butter, cream and fruit. The zingy 100 per cent cacao bar from Peru has a distinct red-wine flavour, followed by a rip roaring bitter edge. The "milky chocolate" is creamy, soft, and unctuous as a royal butler. Such luxury doesn't come cheap, however: a 21-chocolate box will set you back the best part of £25.

Why create something so expensive, so rarefied? Well, unlike most of us, Ducasse doesn't just see chocolate as comfort food to be shovelled down the neck during the post-lunch slump. To him, chocolate is "mighty and subtle, raw yet refined". It deserves, in short, the same respect and care as the dishes he serves in his restaurant. His aim is simple and laudable: create the best chocolate he can.

Will he hang up his chef's whites soon then? Turn into a full-time Willy Wonka? "Nothing against Roald Dahl, Tim Burton or Johnny Depp," he says, "but, as you can see, I don't wear a red coat nor a top hat." Chocolate's loss, I say.

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