Cut the peeled potatoes into pieces and cook them in salted water until cooked (takes about 20 minutes). Pour off and evaporate, but do not allow to cool. Add the butter and mash the potatoes with a potato masher and work in the butter at the same time. Then add the sour cream and continue pounding vigorously until you have a nice, even potato mixture. Season with salt and pepper and add parsley if you like.