Preheat the oven to 350 F. Place potatoes into a large saucepan, and cover with water. Bring to a boil, let cook for about 20 minutes, until the potatoes are fork tender. Drain and place potatoes into a large bowl.
Mash the potatoes with butter, parmesan cheese, cheddar cheese, 1/2 cup mozzarella cheese, salt, and pepper. Mash until smooth.
Spread the potato mixture into a greased baking dish. Top with the remaining 1/2 cup shredded mozzarella cheese. Bake for 20-25 minutes.