Boil unpeeled potato until soft. Let cool slightly then peel and mash.
Stir in confectioners' sugar into the mashed potato and mix until stiff. On a piece of waxed paper or plastic wrap roll out potato mixture into 1/4 inch thick rectangle. Spread peanut butter over entire area (adjust thickness to suit taste). Roll up jellyroll-fashion wrap with plastic wrap and refrigerate for few hours. Cut into slices to serve.