In a large bowl, beat mashed potato, milk, vanilla and salt with an electric mixer on medium speed for 2 minutes
Mix in powdered sugar, 1 cup at a time, until a dough has formed. Once you've added in 6 cups, if you need more, add in 1 Tbsp increments. Dough should be thick, slightly thicker than cookie dough.
Form dough into a ball and dust with powdered sugar. On a sheet of wax or parchment paper, roll dough into 1/8 inch thick rectangle. Trim to 10x12 inches.
Generously sprinkle powdered sugar over top of dough and place another piece of parchment paper over it. Invert the rectangle of dough and discard the paper that is now on top.
Spread peanut butter evenly over dough. Roll dough tightly, long side to long side using parchment paper to guide it, jelly roll style.
Wrap in parchment paper and freezer for 1 hour.
Cut into 14/ inch thick slices and serve
Leftover candy should be stored in an airtight container in the refrigerator.