Alternately- once you prepare the first side of the sandwich with the butter and Parmesan- you can flip it right into your buttered preheated skillet- cheese side down- and then butter and Parmesan the other piece of bread that is on top. Whatever method works for you- I usually just get it all ready at once.
***I recommend medium to medium low heat to cook so the outside cheese dosen't brown or burn before the inside is melted.