How to Make It
Preheat oven to 400°F. Pierce potatoes with a fork, and bake in preheated oven until tender, about 1 hour.
Cut poatoes in half lengthwise, and scoop out flesh, leaving a 1/4-inch-thick shell. Combine warm potato flesh, cheese, milk, sour cream, buttermilk, chives, butter, salt, and pepper in a medium bowl; mash with a potato masher until creamy and well blended.
Divide mixture among potato shells, and top each with Parmigiano-Reggiano cheese. Bake in preheated oven until heated through and bubbly, 12 to 15 minutes.
Turn oven to broil, and broil until browned, about 2 minutes. Sprinkle with chives.