How to Make It
Beat mashed potato and next 3 ingredients at medium speed with a heavy-duty electric mixer 2 minutes. Beat in 6 cups powdered sugar, 1 cup at a time. Add up to 1 cup powdered sugar, 1 Tbsp. at a time, to form dough.
Gather dough into a ball; dust with powdered sugar. Roll to 1/8 inch thick on parchment paper, and cut into a 12- x 10-inch rectangle. Generously sprinkle powdered sugar over dough, and place a piece of parchment paper over rectangle. Invert rectangle; discard parchment paper on top.
Spread peanut butter over dough. Starting at 1 long side, tightly roll up candy, jelly-roll fashion, using parchment paper as a guide. Wrap in parchment paper, and freeze 1 hour. Cut into 1/4-inch-thick slices, and serve. Refrigerate sliced candy in an airtight container up to 1 week.