Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package directions until al dente.
While the pasta is cooking, combine the grated Pecorino Romano cheese and freshly ground black pepper in a medium bowl. Gradually whisk in about 0.5 cup of the starchy pasta water to create a thick paste.
Reserve about 1 cup of the pasta water before draining the spaghetti. Drain the spaghetti and immediately add it to the bowl with the cheese and pepper mixture.
Toss the pasta vigorously with the cheese mixture, adding more pasta water a tablespoon at a time as needed to create a creamy sauce that coats the noodles. The heat from the pasta will melt the cheese and emulsify the sauce.
Serve immediately, topped with extra Pecorino Romano cheese and black pepper if desired.
Notes
The key to a perfect Cacio e Pepe is the emulsification of the cheese and pasta water. Don't be afraid to use a bit more pasta water if needed to achieve a creamy consistency. Ensure your cheese is finely grated for the best results.