Preheat oven to 375°F. Spray 2-quart casserole with cooking spray; set aside.
Combine corn starch, garlic salt, mustard and pepper in a medium saucepan. Stir in milk until smooth.
Stirring occasionally, bring to a boil over medium heat and boil 1 minute. Remove from heat.
Stir in cheeses until melted. Add macaroni and mix well. Turn into prepared casserole. If desired, sprinkle with crushed cereal and drizzle with melted butter.
Bake uncovered for 25 minutes or until bubbly.
Variation: Stir 1 cup diced ham into cheese sauce with macaroni.