Using a 2- or 2 1/2-inch round cookie cutter (depending on size of tomatoes), cut out bread and mozzarella.
Season tomatoes with salt and pepper. Sandwich 1 tomato slice and 1 mozzarella slice between 2 slices of bread. Repeat with remaining tomatoes, mozzarella, and bread to make 16 sandwiches.
Preheat a grill to medium, or heat a grill pan over medium heat. Brush outsides of bread with butter. Grill sandwiches until golden brown and mozzarella is soft but not completely melted, about 5 minutes per side.