With a butter knife, spread butter on one side of each slice of bread. Place 2 slices, buttered side down, on work surface. Divide cheese evenly on both slices. Top with remaining slices, buttered side up.
Heat a large skillet over low heat until warm. Add sandwiches and cover; cook, turning once with a spatula, until the bread is golden-brown and the cheese has just melted, 5 to 8 minutes per side. Slice and serve immediately.
Cook's Notes
You can vary this recipe by adding sliced red onion, sliced tomato, ham, cooked bacon, mustard, or chutney.