Preheat the oven to 400 degrees F. Coat a 13 by 9 by 2-inch baking dish with the 1 tablespoon of room temp 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 with a potato masher. Mix in the melted butter and milk. Mix in the mozzarella, 3/4 cup of the Parmesan and pancetta. Season, to taste, with salt and pepper and add in parsley. Transfer the potatoes to the prepared baking dish. Stir the bread crumbs and remaining 1/4 cup of Parmesan in a small bowl to blend. Sprinkle the bread crumb mixture over the mashed potatoes. Recipe can be prepared up to this point 6 hours ahead of time; cover and chill.
Bake, uncovered, until the topping is golden brown, about 20 minutes.