1. Preheat oven to 375oF. Grease 2 quart casserole dish.
2. Combine cornstarch, salt, mustard and pepper in medium saucepan. Stir in evaporated milk, water, and butter. Cook over medium-heat, stirring constantly, until mixture comes to a boil. Boil for 1 minute. Remove from heat. Stir in 1 1/2 cups cheese until melted. Add macaroni; mix well. Pour into prepared casserole dish. Top with remaining cheese.
3. Bake for 20 to 25 minutes or until cheese is melted and light brown.