Preheat oven to 425 degrees F. Spread butter on one side of each slice of bread and place 4 slices of bread, butter side down, on a baking sheet. Top each with a slice of cheese then spread a tablespoon of pesto over the cheese and top with another slice of cheese. Top each with the other slice of bread, butter side up. Bake for 4 minutes then remove from oven and turn sandwiches over and bake for another 4 minutes. Remove and serve.