How to Make It
Preheat the oven to 425°. Butter a 7-by-11-inch baking dish. In a saucepan, cover the potatoes with water and bring to a boil over moderately high heat. Salt the water and cook the potatoes until tender, about 12 minutes; drain. Shake the potatoes over moderately high heat until very dry. Remove from the heat.
Mash the potatoes, stirring in the milk. Season with salt and pepper. Fold in the mortadella and mozzarella, then spread the potatoes in the prepared baking dish. Dot the top with the butter and bake in the upper third of the oven for about 12 minutes, or until bubbling. Turn on the broiler and broil for about 2 minutes, or until crusty and browned on top. Let rest for 5 minutes before serving.