Place peeled potato in a large bowl. Begin mashing while slowly adding powdered sugar. Add enough sugar to make a stiff dough. (Work fast as the dough hardens as it cools.)
Sprinkle some of the powdered sugar on waxed paper and roll out the dough thinly (about the thickness of pie crust is good). Spread peanut butter evenly over the dough and roll into a log shape. Cut into slices and chill before eating.