Preheat oven to 300 degrees. Spread butter over one side of each slice of bread.
Arrange bread, butter side down, on a piece of plastic wrap or wax paper. Spread each slice of bread with mustard. Arrange cheese over 4 slices of bread, and sandwich together with remaining bread, butter side out.
Set a large cast-iron skillet over medium heat. Cook sandwiches in batches in skillet. Using a spatula, press down firmly on sandwiches. Cook until bread is golden brown, 2 to 3 minutes. Reduce heat to medium low. Turn sandwiches over, press down with spatula, and cook until bread is golden and cheese is starting to melt, about 2 minutes more. Transfer to a baking sheet, and place in oven. Bake until cheese is completely melted. Serve immediately.