In a large bowl, beat mashed potatoes, milk, vanilla, and salt with an electric mixer on medium speed for 2 minutes.
Add powdered sugar, 1 cup at a time, to form a dough shape. If you need more after adding 6 cups, add 1 more tablespoon. The dough should be slightly thicker and longer than cookie dough.
Form the dough into a ball and sprinkle powdered sugar on it. On waxed paper or parchment paper, roll dough into a 1/8-inch rectangle. Try 10 x 12 inches maximum.
Usually sprinkle icing sugar over the dough and place another piece of baking paper on top. Turn the dough rectangle over and discard the top layer (the layer you used to roll out the dough).
Spread the peanut butter evenly over the dough. Very slowly roll the dough back and forth, jelly roll style, using parchment paper to guide it. Wrap in crust and cut into 1/4-inch-long slices, freeze for 1 hour and serve. To enjoy! Leftover sugar should be stored in an airtight container in the refrigerator.
