Peel and boil 1 medium potato until very soft
Drain well and mash until completely smooth
Let the mashed potato cool slightly
Add 4 to 6 cups powdered sugar, a little at a time, mixing until a dough forms
Stir in 1 teaspoon vanilla extract if desired
Dust a clean surface with powdered sugar
Roll the dough into a rectangle about 1/4 inch thick
Spread a thin layer of peanut butter over the surface
Roll the dough tightly into a log
Wrap in plastic wrap and chill until firm
Slice into pieces and serve