Preheat oven to 300°F. Heat a large skillet, preferably cast iron, over medium. Butter one side of each slice of bread and place 4 slices in skillet, buttered side down. Top bread with cheese, reduce heat to low, and cook until underside is golden brown and cheese is melted, about 4 minutes.
Top with remaining bread, buttered side up, turn sandwiches and cook until second side is golden brown, about 2 minutes. Transfer sandwiches to a rimmed baking sheet and place in oven to keep warm.