Cook the Pasta. Bring a large pot of salted water to a boil and cook the spaghetti until al dente. Reserve a cup of pasta cooking water, then drain the pasta.
Cook the Pancetta. In a skillet, cook the pancetta until it's crispy and golden. Remove it from the heat.
Combine Eggs and Cheese. In a bowl, whisk together the eggs and grated Pecorino Romano cheese.
Toss the Pasta. Toss the hot, drained pasta with the cooked guanciale or pancetta. Make sure the pasta is well coated.
Add the Egg Mixture. Pour the egg and cheese mixture over the pasta and toss quickly to combine. The heat from the pasta will cook the eggs and create a creamy sauce. If the sauce is too thick, add some reserved pasta cooking water to achieve the desired consistency.
Season and Serve. Season the Spaghetti Carbonara with freshly ground black pepper. Garnish with chopped fresh parsley, if desired. Serve immediately with extra Pecorino Romano cheese on top.