Separate 4 egg yolks from whites and place yolks in a large mixing bowl. Grate Parmesan cheese fresh from a block. Cut bacon into ½-inch pieces. Measure out all ingredients before starting because everything moves quickly once you begin cooking.

Heat a large skillet over medium heat. Add bacon pieces and cook for 8-10 minutes, stirring occasionally, until golden brown and crispy. Use a slotted spoon to transfer cooked bacon to a paper towel-lined plate. Leave about 1 tablespoon of bacon fat in the pan, drain the rest. Turn off heat and let pan cool slightly.

While bacon cooks, bring a large pot of salted water to rolling boil. Add spaghetti and cook until al dente according to package directions, usually 8-9 minutes. Before draining, use a measuring cup to save 2 cups of the starchy pasta water. Drain pasta but do not rinse.

In your large mixing bowl with egg yolks, add most of the grated Parmesan (reserve some for topping) and lots of freshly ground black pepper. Whisk together until smooth and well combined. The mixture should be thick and creamy. This is your carbonara sauce base.

Make sure your skillet with bacon fat is off the heat and has cooled for about 30 seconds. The pan should be warm but not blazing hot. This temperature control is critical for preventing scrambled eggs in your carbonara.

Add the hot drained pasta to the cooled skillet with bacon fat. Toss pasta to coat it in the bacon fat for extra flavor. Add the crispy bacon pieces back to the pan and toss again to distribute evenly throughout the pasta.

Add ½ cup of the reserved hot pasta water to the skillet with pasta and bacon. Toss to combine. The pasta water lowers the temperature further and adds moisture, preparing the pasta for the egg mixture without scrambling.

Immediately pour the egg yolk and Parmesan mixture over the pasta. Using tongs, toss everything constantly and vigorously for 1-2 minutes. The residual heat from the pasta will cook the eggs into a silky, creamy sauce. Keep tossing to prevent any eggs from setting.

If the sauce seems too thick, add more reserved pasta water a few tablespoons at a time while continuing to toss. If it seems too thin, keep tossing and the sauce will thicken as the eggs temper. The sauce should coat the pasta strands like silk, glossy and smooth, not gloppy or watery.

Immediately divide carbonara among serving plates or bowls. Top with extra grated Parmesan cheese and more freshly ground black pepper. Garnish with fresh parsley if desired. Serve right away while hot and creamy, as the sauce will continue to thicken as it sits.