Dice guanciale into ¼-inch pieces. Heat a large skillet over medium-low heat and add the guanciale, cooking it for 10-15 minutes, stirring occasionally, until it becomes crispy and golden brown. Reserve the rendered fat.

If using garlic, add freshly minced garlic to the skillet with the rendered fat and sauté for about 30 seconds until fragrant.

In a medium bowl, whisk together the large eggs, grated Pecorino Romano, Parmesan cheese, and freshly ground black pepper until smooth and creamy.

Fill a large pot with salted water and bring it to a boil. Add spaghetti and cook until al dente, approximately 8-10 minutes, reserving 1 cup of pasta water before draining.

In the skillet with guanciale, add the hot drained spaghetti while still over low heat. Remove from heat and pour in the egg and cheese mixture, tossing quickly to coat the pasta.

Gradually add reserved pasta water until the sauce reaches desired creamy consistency.

Toss in remaining guanciale for crunch and flavor. Season with additional black pepper and extra cheese as desired. Serve immediately.