Add butter (and garlic if using minced garlic -- garlic powder can wait until step 2) to a frying pan or skillet, and cook over medium heat in a frying pan until butter has melted.
Add leftover spaghetti, a half cup of the shredded mozzarella and half of the parmesan to the melted butter and garlic powder (if using).Cook on high heat for 8-10 minutes, stirring often.
Pour the remaining cheese on top and cover, cooking an additional couple of minutes to melt the cheese, and then serve!