Real carbonara has no cream - that is an addition that took hold outside Rome and stayed there. No mushrooms, no garlic, no bacon either. It has four things: pasta, guanciale, eggs and pecorino romano - plus the cooking water, which is the fifth and the most important.

For four people you need 400 grams of spaghetti, 150 grams of guanciale, four yolks plus one whole egg and 100 grams of pecorino. The water is three to four litres with 30 to 40 grams of salt, and no oil goes into it - oil stops the sauce from clinging to the pasta.

The step where everything falls apart is one and always the same: the pan must be off the heat when the egg mixture goes in. If the pan is hot, you get scrambled eggs with spaghetti. The residual heat from the pasta is enough to thicken the cream without cooking the eggs - and it has to be stirred constantly and vigorously, adding cooking water gradually.

The guanciale is cut into strips and fried slowly, without a drop of oil, until crisp outside and soft inside. The fat that renders out is not waste - it is half the sauce. Chef Massimo Bottura says it plainly: the key to the dish is an emulsion of guanciale fat, egg and the starch from the pasta.

If you have no guanciale, pancetta works, though it is more neutral in flavour. Martín Berasategui recommends cured Iberian pork cheek as the closest substitute. Pecorino can be swapped for parmigiano - that is the only concession the recipe tolerates without becoming something else.

There are several versions of its origin. One links it to the carbonari, the nineteenth-century secret society that cooked simply in the Apennines with eggs, sheep's cheese and pasta. Another points to black pepper resembling coal. A third cites Ippolito Cavalcanti, who published the recipe in his 1839 book.

Four ingredients, three litres of water and one moment when it has to come off the heat. Everything else is adding things nobody asked for.