Bring a large pot of salted water to a boil. Cook pasta until al dente according to package directions. Drain and set aside, reserving ½ cup pasta water if you like.

Whisk cornstarch with 2 tbsp of the milk until smooth. In a saucepan over medium heat, melt butter. Add remaining milk and the cornstarch slurry. Whisk constantly until bubbling and thick, about 2 to 3 minutes. Remove from heat. Stir in cheddar and cream cheese until smooth. Season with salt, pepper, garlic powder, and mustard powder.

Add cooked pasta to the sauce. Fold together until well coated. Adjust consistency with a splash of milk or pasta water if needed.

Spoon into bowls, top with a crack of black pepper, and serve immediately.