Cook the spaghetti according to package instructions in large pot filled with salted water.
While the spaghetti is cooking, cook the chopped pancetta or bacon in a large pan with 1 or 2 whole cloves of garlic and 1 tbsp of butter until the fat is rendered, then discard the garlic. If using bacon, discard all by 1 -2 tablespoons of fat.
In a small bowl mix 1 whole egg and 3 egg yolks (reserve the whites for another use or freeze them) and freshly grated parmesan cheese until blended.
Once the pasta is cooked, drain it while reserving ½ cup of pasta water. Add the pasta directly to the pan with pancetta and about half of the reserved pasta water. Stir to allow the pasta to be coated with the sauce, then take the pan off the heat.
Pour the egg and Parmesan mixture into the spaghetti while tossing it with a pair of tongs the entire time. Toss quickly not to let the eggs to curdle. Then taste and season with more salt if needed and pepper as well as more grated parmesan if desired. You might want to add the rest of the reserved pasta water to make it saucier.