This potato dish makes a really convenient side dish for a festive dinner, since it can be assembled in baking dish, covered, and refrigerated for up to 1 day. Uncover and bake as directed below.
Preheat oven to 375°F. Butter a 13x9x2-inch oval or rectangular baking dish.
Place potatoes in a large saucepan and add enough cold salted water to cover them by 1 inch. Bring water to a simmer over high heat, then reduce heat to medium and simmer for about 15 minutes, or until potatoes are tender.
Drain potatoes in a strainer and set strainer over hot saucepan to evaporate excess steam. Press hot potatoes through a potato ricer or food mill.
Put potatoes in saucepan, and reduce heat to low, then mix in 3 tablespoons of butter.Meanwhile, heat a heavy large skillet over medium-high heat. Add bacon and sauté for about 6 minutes, or until crisp. Using a slotted spoon, transfer bacon to a plate lined with paper towels and reserve 2 tablespoons of pan drippings in a small bowl.
Fold warm milk into mashed potatoes. Fold in reserved pan drippings and bacon, sour cream, and mascarpone cheese. Season to taste with salt and pepper. Transfer potato mixture to prepared dish. Sprinkle Parmesan cheese over.
Bake for about 30 minutes, or until potatoes are heated through, slightly puffed, and top is light brown. Let stand 10 minutes and serve.