Cook pasta: Bring a large pot of water to a boil, add salt generously. Cook macaroni according to the packet's instructions. Reserve about 1 cup of pasta water. Drain well.
Melt cheese: Shortly before the pasta is cooked, heat a large saucepan. Add grated cheese, cream cheese, mustard, some salt, and pepper. Melt cheese on low heat, constantly stirring, for about 2 minutes. Don't overheat the mixture.
Combine: Add drained pasta and about ¼ - ½ of the reserved pasta water (as needed for a creamy consistency) and stir to combine. Adjust the taste with more salt and pepper if necessary. Serve immediately.
Any other short pasta shape will be fine.
Don't forget to reserve some pasta water before draining the noodles.
You can use other cheese sorts as well - for example, Gouda, Fontina, Swiss, Provolone, and so on.