Boil a pot of salted water and cook the spaghetti until al dente.
Cut the pancetta or guanciale into small cubes.
Cook the pancetta in a skillet over medium heat until crispy.
Whisk together the eggs, Pecorino Romano, and Parmesan cheese in a bowl.
Add hot pasta to the skillet with pancetta, removing from heat.
Pour the egg mixture over the pasta and stir quickly to create a creamy sauce.
Adjust sauce consistency with reserved pasta water, if necessary.
Season with salt and freshly cracked black pepper.
Serve immediately, garnished with chopped parsley.