Preheat a skillet over medium heat until hot but not smoking.

Spread butter evenly on one side of each bread slice. Put one slice in the skillet, buttered side down. Layer half the cheese, about half the pizza sauce, the pepperoni, the remaining sauce, then the rest of the cheese. Top with the second slice of bread, buttered side up.

Cook until the bottom is golden and crisp, 2 to 4 minutes. Flip carefully and continue cooking until the other side is browned and the cheese has fully melted, another 2 to 4 minutes. Press gently with a spatula if needed to help the sandwich heat through.

Remove from the skillet, let rest for a minute, then slice and serve warm.