For 4 servings
Bring a large pot of salted water to a boil and cook spaghetti until al dente according to package directions.
While pasta cooks, fry guanciale in a large skillet over medium heat until golden and crispy, about 8 minutes. Remove from heat.
Whisk together eggs, Pecorino, Parmesan, and black pepper in a bowl until smooth.
Reserve 1 cup of pasta cooking water before draining.
Add hot drained pasta to the skillet with the guanciale (off heat). Toss to coat in the rendered fat.
Pour egg mixture over pasta, tossing rapidly and adding pasta water a splash at a time to create a creamy sauce.
Serve immediately with extra Pecorino and black pepper on top.