Place the butter and sour cream out on a warm counter to get them to room temp before peeling and cutting up the potatoes.
Put the cut potatoes in a large pot with cold, lightly salted water. Bring to a boil over med/hi.
Turn heat down and simmer until the potatoes are soft.
Drain and place the potatoes in a large bowl.
Add the butter and mash with a potato masher until the butter is melted.
Add the sour cream.
Blend with a hand mixer.
If needed, add more sour cream to get the desired consistency.
Season with salt and freshly ground black pepper to taste.
Cover the dish with a lid or foil to keep hot until ready to serve.