Bring a large pot of salted water to a boil. Cook the spaghetti according to the package instructions until al dente. Drain the pasta, but reserve about 1/2 cup of the cooking water.

While the pasta is cooking, in a separate bowl, whisk together the eggs, grated Pecorino Romano cheese, and a generous amount of freshly ground black pepper. Set aside.

In a large skillet, cook the diced guanciale or pancetta over medium heat until it becomes crispy and golden brown, about 4-5 minutes. If you're using garlic, add it to the pan during the last minute of cooking the meat.

Remove the pan from the heat.

Add the drained spaghetti to the skillet with the crispy guanciale or pancetta. Toss to combine, ensuring the pasta is well coated with the rendered fat from the meat.

Quickly pour the egg and cheese mixture over the hot pasta and toss vigorously to create a creamy sauce. If the sauce is too thick, add some of the reserved pasta cooking water a little at a time until you reach your desired consistency.

Season with salt to taste, but be cautious as the cheese and meat are already salty.

Serve the spaghetti carbonara hot, garnished with additional Pecorino Romano cheese, freshly ground black pepper, and chopped fresh parsley if desired.