Preheat oven to 375 degrees. In a medium saucepan combine cornstarch, salt and pepper- stir in milk.Add butter. Stir Constantly and bring to boil over medium high heat and boil one minute. Remove from heat.Stir in 3 cups of cheese until melted. Add macaroni. Pour into a greased 4 quart casserole and sprinkle with the remaining cheese. Bake uncovered 25 minutes or until brown.