Dice the guanciale into small pieces and place in a cold skillet over medium-low heat, cooking until crispy, about 10–15 minutes.

If using garlic, add olive oil to the guanciale fat and sauté minced garlic for 1–2 minutes until fragrant. Remove from heat.

In a bowl, whisk together eggs, pecorino cheese, and parmesan until smooth. Season with black pepper.

Boil salted water and cook spaghetti according to package instructions until al dente, about 8–10 minutes. Reserve 1 cup of pasta water before draining.

Return spaghetti to the skillet with guanciale, then pour the egg mixture over it, tossing quickly to coat.

If the sauce is too thick, gradually add reserved pasta water until desired consistency is reached.

Fold in remaining crispy guanciale and adjust seasoning with black pepper. Serve immediately with extra cheese on top.