Put butter on one side of a slice of bread, placed it butter side down on a lightly greased baking sheet. Add two pieces of American cheese and butter the second piece of bread on one side, with horseradish mustard on the other side. Put the bread on top of the cheese, butter side up , and put the pan in the oven. Put the pan in a preheated 400 degree oven on the middle rack. Let the sandwiches cook for about 15 minutes and then flip them over. Continue to cook the sandwiches until the edges begin to brown.