Preheat oven to 375°F.

Cook macaroni about 7 minutes. Drain and set aside.

In medium saucepan, melt butter, Stir in corn starch.and mustard, and whisk milk in slowly until smooth. Cook over medium heat until sauce reaches a boil and thickens. Add pepper, salt, Swiss, American, and 1 cup Cheddar and Stir until smooth.

Stir macaroni into sauce and Pour into 13x9 casserole. Top with 1 cup Cheddar. Bake uncovered 30 minutes until lightly browned and bubbly.