Preheat the oven to 375°F. Butter a 13x9x2-inch oval or rectangular baking dish.

Place the potatoes in a large saucepan and add enough cold, salted water to cover them by an inch. Bring the water to a simmer over high heat then reduce the heat to medium and simmer for about 15 minutes, or until the potatoes are tender.

Drain the potatoes in a strainer and set the strainer over the hot saucepan to evaporate the excess steam.

Press the potatoes through a potato ricer or food mill. Put the potatoes in the saucepan, and reduce the heat to low, then mix in 3 tablespoons of the butter.

Meanwhile, heat a large, heavy skillet over medium-high heat. Add the pancetta and the remaining 1 tablespoon butter and sauté for about 6 minutes, or until crisp. Using a slotted spoon, transfer the pancetta to a plate lined with paper towels and reserve 2 tablespoons of the pan drippings in a small bowl.

Fold the warm milk into the potatoes. Fold in the reserved pan drippings and pancetta, sour cream and mascarpone cheese. Season to taste with salt and pepper. Transfer the potato mixture to the prepared dish. Sprinkle with Parmesan cheese.

Bake for about 30 minutes, or until the potatoes are heated through, slightly puffed and the top is light brown. Let stand 10 minutes and serve.