Add the mashed potatoes, butter, and salt to a large mixing bowl. Beat with a hand mixer until smooth.
Add 1 cup of powdered sugar at a time to the potatoes and mix until combined. Repeat. You want to use enough sugar so that it's not overly dry (I've used up to 5 ½ cups before). You should be able to pick up a piece and have it stick together in your hand when pressed - like cookie dough. (If the potato dough is too dry you can add a little milk.)
Sprinkle a cutting board or parchment paper with some powdered sugar. Then, form the potato dough into a rectangle and sprinkle the top with more sugar.
Use a rolling pin to roll out the dough into a ¼-inch thick rectangle (as best you can for the shape). You can add parchment paper on top to help roll out or sprinkle with more powdered sugar as needed. Do this step quickly so the dough does not dry out.
Spread the peanut butter evenly over the dough, leaving a 1-inch along one of the long sides. Spread the peanut butter all the way to the edges of the other 3 sides. Then, starting with long with peanut butter, roll the dough up tightly into a log ending seam side down. Trim off any excess on the ends so you start with a nice edge.
Wrap the log in plastic wrap and place it in the fridge for about 20-30 minutes.
Remove the candy from the fridge and unwrap it. Cut the log into slices about ¼ to ½-inch thick and enjoy!
Notes
Nutritional information will vary based on how thick you cut the slices since this can change the recipe yield.
To make the mashed potatoes: pierce a medium Russett potato all over with a fork. Wrap it in a damp paper towel and microwave for 10 minutes, flipping halfway through the cooking time until fork tender. (Be careful! It will be very hot.) Remove the skin from the potatoes and mash until smooth. Let the potatoes cool to room temperature. The amount of potato I used was ½ cup, not packed but slightly heaping.
If left in the fridge too long you want it to come closer to room temp before cutting so they don’t crumble too much.