Preheat oven to 425°. Rinse potatoes and dry. Pierce each potato several times with a fork. Rub each potato lightly with butter. Place potatoes directly on middle rack of oven and bake until tender, about 1 hour to 1 hour and 15 minutes. The potatoes should yield when squeezed lightly wearing an oven mitt.

Remove potatoes from oven. Wearing a mitt in one hand, slice potatoes in half lengthwise. Scoop out hot potato pulp into a large mixing bowl; reserve potato shells. Be careful not to tear shells.

Using electric beaters, beat baked potato pulp on low speed, adding cream cheese, sour cream, half and half, 4 tablespoons butter, salt, and pepper. When potatoes begin to get smooth, increase speed to high and continue beating until quite smooth. Adjust consistency and flavor with milk, cream cheese, butter, and sour cream, as needed. The texture should be smooth and spoonable, but not runny. Season to taste with salt and pepper.

Spoon potato mixture back into shells, creating small peaks. Sprinkle each potato with Parmesan cheese. Melt remaining 2 tablespoons butter and drizzle over potatoes. Lower oven temperature to 325° to 350°. Return potatoes to oven and bake until cheese melts and edges are golden, about 20 minutes.

If desired, stuffed potato shells may be made ahead and refrigerated until just before serving time. Finish baking, allowing 30 to 35 minutes for reheating and browning.