Cook spaghetti in well-salted boiling water until al dente. Reserve 1 cup of the pasta water before draining.

In a mixing bowl, whisk together egg yolks, Pecorino Romano, and Parmesan to form a thick paste.

Add a spoonful of rendered fat from the pancetta/guanciale and a generous amount of black pepper to the egg and cheese mixture. Mix well.

In a cold pan, add pancetta or guanciale and cook over medium heat until crispy and browned. Remove some fat if needed, then set meat aside.

Remove pan from heat and let cool slightly. Add cooked pasta to the pan (still off heat).

Quickly pour in the egg mixture along with a dash of reserved pasta water. Mix vigorously until the sauce emulsifies and becomes creamy—avoid scrambling the eggs.

Stir in additional cheese and pasta water as needed to reach desired consistency. Fold in the cooked pancetta/guanciale.

Serve immediately topped with more black pepper and grated cheese.