In a large pot, add the potatoes and cover amply with cold water. Bring the water up to a boil and reduce the heat so it simmers. Add a generous pinch of salt to the water and allow the potatoes to cook until tender when pierced with the tip of a knife (about 20 to 25 minutes). Drain the potatoes in a colander and, while still hot, put them through a food mill. Return the empty pot to the heat and add the milk. Bring the milk to a simmer and gingerly add the potatoes back into the pot. Season with salt and pepper and gently mix in the butter and sour cream. Do not over mix (over mixing makes gummy potatoes). Taste for seasoning. Remove from the heat. When ready to serve, use a whisk (or an electric mixer fitted with the whisk attachment) to whip the heavy cream until fairly firm. With a rubber spatula, gently fold the whipped cream and the Parmesan into the mashed potatoes. Taste for seasoning. Serve immediately.