Bring a large pot of water to a boil and salt it lightly, using less salt than usual because the guanciale and Pecorino Romano are already salty.

In a bowl, whisk together the four egg yolks, whole egg, finely grated Pecorino Romano, and most of the black pepper until a thick paste forms.

Place the guanciale in a cold skillet and cook over medium-low to medium heat for about 6–8 minutes, turning or stirring as needed, until the fat renders and the meat is golden and crisp. Turn off the heat and leave the guanciale in the skillet.

Cook the spaghetti according to the package timing until just al dente. Before draining, reserve at least 250 ml, or 1 cup, of the starchy pasta water.

Add the drained spaghetti to the skillet with the guanciale and toss well to coat it in the rendered fat. Let the pan cool for about 30 seconds so it remains warm but is no longer scorching hot.

Remove the skillet from direct heat and add the egg-and-cheese mixture. Toss continuously, adding the reserved pasta water one or two tablespoons at a time until the sauce becomes smooth, glossy, and creamy.

Taste and add more black pepper or Pecorino Romano as desired. Serve immediately while the sauce is silky and the guanciale is crisp.