Prepare the sauce while the pasts is cooking to ensure that the spaghetti will be hot and ready when the sauce is finished; it is very important that the pasts is hot when adding the egg mixture, so that the heat of the pasta cooks the raw eggs in the sauce.
Bring a large pot of salted water to a boil. add the pasta and cook for 8-10 mins or until tender yet firm (al dente). Reserve 1/2 cup of pasta water, and drain the pasta well.
Meanwhile, heat the olive oil in a deep skillet over medium heat. Add the pancetta or bacon and saute for about 3 minutes. until the bacon is crisp and the pat is rendered. Toss the garlic into the fat and saute for less then 1 minute to soften.
Add the hot, drained spaghetti to the pan and toss for 2 minutes to coat the pasta in the bacon fat. Beat the eggs and Parmesan together in a mixing bowl, stirring well to prevent lumps. Remove the pan from the heat and pour the egg/cheese mixture into the pasta, whisking quickly until the eggs thicken, but do not scramble.
Thin out the sauce with the reserved pasta water, until it reaches desired consistency. Season the carbonara with several turns of freshly ground black pepper and taste for salt. use additional grated parmesan on top of servings and garnish with parsley.