Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until tender yet firm to the bite, about 12 minutes. Drain, saving about 1 cup of cooking water.
Meanwhile, place guanciale in a large skillet and cook over medium-high heat, turning occasionally, until crisp, about 3 minutes. Add pasta water, cooked spaghetti, and grated Pecorino Romano cheese. Mix well.
Distribute pasta amongst 4 bowls and serve with freshly ground black pepper and extra Pecorino Romano cheese.
