First, warm your serving dishes in a low oven or in the microwave. Heat the oil gently in a large, shallow pan. Fry the guanciale or pancetta and garlic for 10 mins, or until all the fat has rendered off and the meat is golden and crisp. Remove and discard the garlic clove, then turn off the heat.
Bring a large pan of salted water to a simmer and cook the pasta until it’s al dente (about a minute less than the pack instructions). Meanwhile, whisk 2 egg yolks in a small bowl with a pinch of salt.
Using kitchen tongs, lift the pasta from the water into the pancetta pan along with any dripping water. Use a wooden spoon to stir it into the rendered fat. If the pan looks dry, add a small ladleful of pasta water and mix it in. Keep adding until you see a little pasta water at the bottom of the pan – you’ll be surprised how much will be absorbed.
Working quickly, tip the beaten yolks in with the pasta and stir vigorously. Rinse the yolk bowl out with a little more pasta water and pour that in too. Add most of the parmesan and beat again. If at any time it’s becoming claggy or starting to scramble, pour in a little more water. If you’ve added too much, stir the pasta over the lowest heat for a few moments. You’re aiming for a smooth sauce that is the consistency of double cream. Season with a couple of pinches of ground black pepper and taste for salt.
Transfer the pasta to the warmed serving dishes, scatter over the remaining parmesan and nestle the last two egg yolks on top, if using. Season them with some salt and pepper, and drizzle over the truffle oil, if you have it, just before serving.