Combine macaroni, butter, and egg in a large mixing bowl.
Dissolve salt and mustard in hot water; stir into macaroni.
Mix in cheese, reserving enough for topping.
Lightly spray a 2-qt casserole dish with no-stick cooking spray; pour in macaroni; pour milk evenly over macaroni; top with croutons and remaining cheese.
Bake at 350 for 45 minutes or until custard is set and top is crusty.
This macaroni and cheese is best baked for about 25 minutes, removed from the oven, topped with croutons and cheese, then returned to the oven to finish baking. Doing this keeps the topping a nice (not overdone) golden-brown.
NOTE: 8 ounces elbow macaroni equals 2 cups before cooking. I often use rounded cups of dry macaroni when making this recipe.