Prepare the Mixture: In a large bowl, beat the mashed potato, milk, vanilla extract, and salt with an electric mixer on medium speed for 2 minutes.
Incorporate Powdered Sugar: Gradually mix in powdered sugar, 1 cup at a time, until a dough forms. Add more powdered sugar if needed to achieve a dough slightly thicker than cookie dough.
Roll Out Dough: Form the dough into a ball, dust with powdered sugar, and roll out on wax or parchment paper into a 10x12-inch rectangle.
Add Peanut Butter: Sprinkle powdered sugar over the dough, place another parchment paper on top, invert, and spread peanut butter evenly over the dough.
Roll and Chill: Roll the dough tightly from long side to long side using parchment paper, wrap, and freeze for 1 hour.
Slice and Serve: After freezing, slice into ¼-inch thick pieces. Store in an airtight container in the refrigerator.
