Preheat oven to 350 degrees. Boil 8 ounces of elbow pasta according to package directions. In a medium-size pot over medium heat add butter and flour. Stir continuously while for about 1 minute. The flour paste should become fragrant. Slowly add milk in thirds and stir until it has thickened before adding the next portion of milk. Shred cheese and slowly add the cheese to the milk mixture.
Make sure all cheese melts completely before adding the next portion of cheese. Add dry mustard powder and paprika and stir well. Drain pasta. Stir pasta into cheese sauce. Place pasta into a large 2 quart baking dish. Bake for approximately 20 minutes. If desired turn oven to broil, and allow the casserole to brown a moment or two before serving.