Preheat oven to 400ºF. Wash and prick potatoes and bake for about 35 minutes, or until they can be pierced with a fork. Leave oven on and grease a 12-cup muffin tin.
While potatoes are hot, peel them. Put through a ricer or shred using a grater. In a bowl, combine potatoes, butter, salt, garlic powder, and pepper and mix until the butter has melted.
Fill each muffin cup about halfway with potato mixture. Place 1 piece of mozzarella in each cup. Divide the pepperoni evenly between cups and top with remaining potatoes. Sprinkle bread crumbs on top.
Return to oven and bake for 10 to 12 minutes. Unmold immediately and serve.