Preheat the oven to 400°.
Coat a baking dish with 1 Tbsp butter and set aside.
Cook the potatoes in a large pot of boiling salted water until they are very tender, about 15 minutes.
Drain; return the potatoes to the same pot and mash well.
Mix in the milk and melted butter.
Mix in the mozzarella and 3/4 of the Parmesan.
Season, to taste, with S&P.
Transfer the potatoes to the prepared baking dish.
Stir the bread crumbs and remaining Parmesan in a small bowl to blend.
Sprinkle the bread crumb mixture over the mashed potatoes.
Cover and chill. *Note: Recipe can be prepared up to this point 6 hours ahead of time
Bake, uncovered, until the topping is golden brown, about 20 minutes.
Recipe and Picture courtesy of Sean Bardwell at http://www.sbque.com/