Pierce potatoes with a fork; bake at 400° for 1 hour or until tender. Cut potatoes in half lengthwise. Scoop out flesh, leaving about a 1/4-inch-thick shell. Combine flesh, milk, sour cream, and buttermilk; mash with a potato masher to desired consistency. Stir in fontina and next 4 ingredients (through pepper). Divide mixture evenly among shells. Place potatoes in a single layer on a baking sheet; sprinkle potatoes evenly with Parmigiano-Reggiano. Bake at 400° for 12 minutes or until heated.
Broil potatoes 2 minutes or until browned and bubbly.