In a large bowl, mix the mashed potato, milk, vanilla, & salt with an electric mixer on medium speed for 2 minutes. Add 6 cups of the powdered sugar, 1 cup at a time. Add the last cup of powdered sugar, 1 tablespoon at a time. Once the mixture creates a dough, you have added enough powdered sugar. You may or may not use all of the last <g class="gr_ gr_71 gr-alert gr_gramm gr_inline_cards gr_run_anim Grammar multiReplace" id="71" data-gr-id="71">cup</g> of powdered sugar.
Form the dough into a ball & dust with powdered sugar. Lay the ball of dough on a large piece of parchment paper. Roll dough out to 1/8 inch thickness. Cut the sides to make a 12x10 rectangle.
Sprinkle a generous amount of powdered sugar over the dough. Lay a fresh sheet of parchment paper over the top of the dough. Invert the dough and now discard the parchment paper on top.
Spread the peanut butter evenly over the dough.
Starting from a long side, tightly roll the dough jelly-roll style. Use the parchment paper to help guide the dough. Don't worry if the dough cracks along the way. Just press the dough a little to seal the cracks.
Wrap the roll with a fresh sheet of parchment paper and freeze for 1 hour.
Remove from the freezer & cut into 1/4 inch thick slices. Serve and enjoy!
Store in an airtight container for up to 1 week.