Bring a large pot of slightly-salted water to a boil. Add the macaroni and cook as per package direction or until al dente. Drain and put back into the pot.
Melt the butter in a saucepan over medium heat. Add the flour and cook for 1 minute. Whisk in the milk in a thin stream. Add the garlic powder, salt, and pepper. Bring to a simmer while whisking occasionally for 3 minutes.
Remove from the heat and whisk in the American cheese and 1 ½ cups cheddar cheese. Whisk a little of the cheese sauce into the egg to temper. Whisk the egg mixture back into the cheese sauce.
Stir in the sauce to the macaroni. Coat insides of standard muffin cups or individual ceramic ramekins greased with nonstick cooking spray. Divide the macaroni among the cups. Top each with 1 tablespoon of the remaining cheddar cheese.
Bake in the oven using the grill function for 20 minutes until slightly browned. Cool slightly before running a thin spatula around the edge and spooning out of the pans.