Combine mashed potatoes, butter, and one cup of sugar in a large bowl and use an electric mixer to stir until combined.

Add remaining sugar, 1 cup at a time and stirring until combined after each addition. Once you've added 6 cups of powdered sugar, check the consistency. If the dough is not moldable in your hands and can't be rolled into a ball, continue to add sugar until it is firm.

Stir in vanilla extract.

Refrigerate for at least 30-60 minutes (if you chill longer it may become too firm and brittle and will just need to sit at room temperature for 10-15 minutes until it is pliable).

Once chilled, divide dough into two pieces and place one piece on a clean surface that you've generously dusted with powdered sugar. Dust the surface of the dough with additional sugar, and use a rolling pin to roll dough out into a rectangle about ¼" thick (about 8x12"). If your dough is too sticky or falling apart, you may need to add more sugar, re-shape it into a ball, and start over.

Once dough has been rolled into a rectangle, spread evenly with peanut butter, leaving a small amount of space peanut butter-free around the perimeter of the dough.

Starting with the longer side of your rectangle, gently but tightly roll into a log.

Use a harp knife to cut into slices about ¼-½" thick.

Repeat steps 5-8 with remaining half of dough.

Serve and enjoy. Store leftover candy in an airtight container in the refrigerator for up to a week.