The spaghetti should be piping hot when you mix it with the egg and cream mixture. The heat from the spaghetti is the only cooking the egg mixture has. If you were to mix it over heat you risk scrambling the eggs. Timing is everything here. Have everything prepped before you start the spaghetti cooking so all the components of the dish are ready to go as soon as the spaghetti is.

Spaghetti alla Carbonara
based on a recipe by Rick Marzullo O’Connell in 365 Easy Italian Recipes

1 pound spaghetti, cooked to al dente
3 tablespoons Olive Oil, divided
8 ounces Panchetta or good quality bacon, diced into ¼ inch pieces
4 large eggs, beaten
1 cup heavy cream
1 cup grated Parmesan plus more for serving
1 teaspoon black pepper

1. Cook pasta according to package directions.
2. While the pasta is cooking in a large skillet heat 1 tablespoon Olive Oil and fry pancetta until browned and crispy. Drain grease and add back two tablespoons Olive Oil.
3. In a large bowl beat eggs then add cream beating again until well blended. Stir in the Parmesan and black pepper.

4. When Pasta is ready drain and quickly add to bowl with the cream mixture. Toss together until the spaghetti is tossed well and coated with the mixture. Add the bacon and olive oil mixture and toss well again. Serve immediately with additional Parmesan and black pepper on the side.

Source: Cinnamon Spice and Everything Nice