Preheat oven 375 degrees F. Melt butter in saucepan on low heat. Remove from heat, blend in flour, salt and pepper. Add milk, and place back on burner at low heat; stirring constantly until sauce thickens slightly and is smooth. Add cheese, heat until melted, stirring occasionally. Combine cooked macaroni and cheese sauce in a 2-quart oven safe casserole dish. Bake for 20 minutes.