In a large pot, melt butter over medium heat. Add flour and cook one minute, stirring occasionally. Add one cup water and whisk until the mixture is smooth and beginning to thicken. Slowly stir in remaining two cups water and the milk.

Add pasta, salt, garlic powder, mustard powder (if using), and black pepper. Bring to a boil, then lower to a simmer and cook eight to 10 minutes, until pasta is al dente. Remove from heat and stir in the cheeses. Serve immediately.