Bring a pot of generously salted water to a boil and drop the potatoes in. Boil until the potatoes are tender and easily pierced with a fork, about 20-30 minutes. Drain and return to the pot. Mash immediately with a potato masher until smooth.
Add the butter and whip the potatoes with an electric mixer. Add the sour cream and whip until incorporated. Add the heavy cream a little at a time while whipping until smooth. Taste for seasoning and add salt and pepper as desired.