Preheat oven to 375F.

Cook macaroni according to package directions until al dente.

Drain macaroni and set aside.

In a medium saucepan, whisk together cornstarch, salt, dry mustard, and pepper.

Gradually whisk in milk until the mixture is smooth.

Add butter to the saucepan.

Cook over medium heat, stirring constantly, until the sauce thickens slightly and comes to a boil.

Be careful not to scorch the milk.

Remove the saucepan from heat.

Reserve 1/4 cup of grated cheese for topping.

Whisk the remaining cheese into the milk mixture until smooth and melted.

Return the drained macaroni to the pot.

Stir in the cheese sauce, ensuring the macaroni is evenly coated.

Pour the macaroni and cheese mixture into a 9x13 inch casserole dish.

Sprinkle the reserved cheese over the top.

Bake for 25 minutes, or until the macaroni and cheese is hot and bubbly and the topping is golden brown.