Preheat oven to 350 degrees. Cook macaroni according to package directions. Drain and rinse with cold water. Pour into a 3-quart baking dish. In a saucepan, melt butter and stir in flour. Gradually stir in milk and sour cream. Add Velveeta, parmesan cheese, salt, pepper and mustard. Cook over low heat, stirring constantly, until sauce bubbles and thickens. Toss macaroni with cheddar cheese. Pour sauce over macaroni and mix thoroughly. Bake uncovered for 1 hour, or until bubbly and brown.
