Bring a large pot of salted water to boil. Add spaghetti and cook according to package directions until al dente, reserving 1 cup pasta cooking water before draining.
While pasta cooks, heat a large skillet over medium heat. Add diced guanciale and cook 4-5 minutes until crispy and golden, rendering the fat.
In a large bowl, whisk together eggs, grated Pecorino Romano, and black pepper until well combined.
Remove skillet from heat. Add drained hot pasta to the skillet with rendered guanciale and toss to coat with the fat.
Working quickly, add the egg and cheese mixture to the pasta, tossing vigorously with tongs. Add pasta water gradually, 2-3 tablespoons at a time, until sauce is creamy and coats the pasta.
Serve immediately in warmed bowls, topped with additional grated cheese and freshly cracked black pepper.
Notes
The key to success is temperature control - the pasta should be hot enough to cook the eggs gently but not scramble them. Work quickly when combining the hot pasta with the egg mixture. If the sauce seems too thick, add more pasta water gradually.