You can make it ahead, freeze it and then just pull out and bake as many pieces as you want. To do this, place the buttered slices of bread on a baking sheet and freeze. Then place in a plastic storage bag with a piece of parchment paper or wax paper between the slices and store in the freezer. When you are ready to bake the slices, remove from the freezer, top them with the shredded cheese, and bake as directed. You may have to cook them for a minute or so longer.