Preheat the oven to broil. Prepare a 9x13 baking dish with nonstick cooking spray.
Bring a large pot of water to a boil and add the elbow macaroni. Cook for 20 minutes. Drain and rinse with cold water. Transfer the macaroni to the prepared baking dish.
In a small bowl, toss together ½ cup of the cheddar cheese and ½ cup of the white cheddar cheese. Set aside to use for the topping.
Meanwhile, in a large saucepan over medium heat, heat the milk, American cheese, butter, and salt, stirring often until the cheese has melted and the sauce is smooth.
Add the remaining cheeses to the saucepan and heat just until melted. Remove from heat.
Pour the cheese sauce over the macaroni and stir to blend.
Sprinkle the reserved cheeses over the top of the casserole.
Place under the oven broiler and heat for 2 to 3 minutes or until the cheese is beginning to brown slightly.