Boil salted water in a large pot
Cook spaghetti until al dente
Reserve some pasta water
Fry guanciale or pancetta in a pan until crisp
Whisk egg yolks with grated Pecorino Romano and black pepper in a bowl
Drain the pasta
Add the hot pasta to the pan with the guanciale
Remove the pan from heat
Add the egg and cheese mixture to the pasta
Toss quickly to create a creamy sauce
Add reserved pasta water a little at a time if needed
Serve immediately with extra Pecorino Romano and black pepper