Cook potatoes in a large pot of boiling, salted water until just tender, about 25 minutes. Drain. Return potatoes to same pot.
Add sour cream and butter, mash until smooth.
Season to taste with salt and pepper.

Note: Créam fraîche may be substituted for sour cream. Potatoes can be made up to 2 hours ahead. Let stand at room temperature. Rewarm over low-heat, stirring frequently.