Boil a large pot of salted water and cook the spaghetti according to package instructions until al dente.
Chop the pancetta or guanciale into small pieces.
In a skillet over medium heat, cook the pancetta until crispy, about 5-7 minutes.
In a mixing bowl, whisk together the eggs, Pecorino Romano, Parmesan, and black pepper.
Once the pasta is done, reserve some pasta water, then drain the spaghetti.
Add the hot spaghetti directly to the skillet with the pancetta, tossing to combine.
Remove the skillet from heat and quickly pour the egg mixture over the pasta, stirring vigorously.
If the sauce is too thick, add reserved pasta water a little at a time.
Season with additional salt and pepper to taste.
Serve immediately, garnished with fresh parsley and extra cheese, if desired.