Dice potatoes into ½" pieces. Place in small to medium pot with enough salted water to cover by 2".
Bring to a boil and cook until tender, about 15-20 minutes. Reserve ½ cup of potato cooking liquid; drain and return potatoes to pot.
Add the butter and sour cream. Mash potatoes until smooth and creamy, adding splashes of reserved potato cooking liquid as needed. Season with salt and pepper. Enjoy!