Boil a large pot of salted water and cook the spaghetti until al dente.
Dice the pancetta or guanciale into small pieces.
In a large skillet, cook the pancetta over medium heat until crispy.
In a bowl, whisk together eggs and grated Pecorino Romano cheese.
Add the hot spaghetti to the skillet with pancetta.
Quickly mix in the egg and cheese mixture.
Add reserved pasta water as needed for creaminess.
Season with salt and freshly cracked black pepper.
Serve immediately, garnished with parsley.