Bring a large pot of salted water to a boil and cook the spaghetti until just al dente. Before draining, reserve 1 1/2 cups of the pasta cooking water.
While the pasta cooks, cook the bacon in a large skillet over medium heat until browned and crisp. Transfer the bacon to a plate while leaving about 1 tablespoon of the rendered bacon fat in the skillet.
Whisk the heavy cream, milk, and cornstarch together until completely smooth. Pour the mixture into the skillet and cook over medium-low heat for 2 to 3 minutes, stirring until it begins to lightly thicken.
Add the drained spaghetti and bacon to the skillet and toss thoroughly so the pasta is coated in the creamy sauce.
Remove the skillet from the heat. Add the Parmesan and black pepper, then toss vigorously while adding reserved pasta water a little at a time until the sauce becomes glossy, silky, and loose enough to cling to every strand.
Stir in the salt and serve immediately, adding a small splash of reserved pasta water if the sauce thickens before serving.
