Arrange 8 slices of bread on a board and spread each with butter.
Flip 4 of the slices and pile ¼ cup of cheese on each slice. Layer each with 1 tablespoon of pizza sauce, 8 pepperoni slices, another tablespoon of pizza sauce and ¼ cup cheese. Top with remaining bread slices, buttered side up.
Preheat a large skillet over medium heat. Carefully transfer one sandwich to the skillet and cook until bread is golden and cheese is starting to melt, 2-3 minutes. Flip and cook another 2-3 minutes, until cheese is fully melted. Repeat with remaining 3 sandwiches. Serve with extra sauce.