Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally.

While pasta cooks, immediately heat a nonstick pan over medium heat. Add guanciale and cook until tender, about 5 minutes. Add 1 ladle of pasta cooking water, stir, and cover pan. As pasta cooks, add another ladle of cooking water to the pan with the guanciale to create a sauce.

At about 10 minutes of pasta cooking time, reserve a cup of cooking water and drain pasta. Add spaghetti to the pan with the guanciale; toss, adding reserved cooking water if necessary. Turn off heat. Add 1/3 the Pecorino Romano cheese to the pasta and toss until creamy.

Divide pasta between 4 plates. Top each with remaining Pecorino Romano cheese and black pepper. Serve immediately.