Bring a large pot of salted water to a rolling boil. Add spaghetti and cook according to package directions until al dente. Reserve about 1 cup of pasta water before draining.

While the pasta cooks, heat a large skillet over medium heat. Add diced guanciale (or pancetta) and cook until crispy and the fat has rendered. Remove the guanciale with a slotted spoon and set aside, leaving the rendered fat in the skillet.

In a medium bowl, whisk together the egg yolks, whole egg, grated Pecorino Romano (and Parmigiano Reggiano, if using), and freshly ground black pepper until well combined and creamy.

Once the spaghetti is al dente, drain it, but do not rinse. Immediately add the hot spaghetti to the skillet with the rendered guanciale fat. Toss to coat.

Quickly pour the egg and cheese mixture over the hot spaghetti in the skillet. Add a splash of the reserved pasta water (about 1/4 cup) and toss vigorously with tongs, constantly moving the pasta to prevent the eggs from scrambling. The heat from the pasta will cook the eggs into a creamy sauce. Add more pasta water a tablespoon at a time if the sauce is too thick, until it reaches your desired consistency.

Stir in the crispy guanciale. Taste and adjust seasoning if necessary (Pecorino is salty, so usually no extra salt is needed).

Serve immediately, garnished with extra black pepper and grated Pecorino Romano if desired.