Cook the pasta. Bring 2 quarts water to a boil in a 3-quart Dutch oven or medium saucepan. Add 1 tablespoon kosher salt, then add the pasta and cook until fully tender. Scoop out 1 cup pasta water, then drain.
Start the sauce. Return the drained pasta to the pot. Add the cream cheese, Parmesan, any optional cheese scraps, and ½ cup of the reserved pasta water.
Stir until creamy. Set the pot over medium heat. Stir as the cheeses melt and the sauce starts to come together.
Cook until glossy. Keep cooking, stirring often, until the sauce thickens and coats the pasta, about 3 to 5 minutes. If the sauce gets too thick, add more pasta water, a splash at a time, until it looks smooth, creamy, and glossy.
Season and serve. Add black pepper, if you like. Let the mac and cheese cool for a few minutes before serving so the sauce can settle and cling to the pasta.