Place the cooled mashed potatoes into a large bowl. Add the butter and beat with an electric hand mixer until smooth. Add the vanilla and stir in until combined.
Slowly add in the powdered sugar 1 cup at a time, fully mixing in the first cup before adding the next. How much you use depends on how watery your mashed potato is as well as humidity, etc.
You want the consistency to be like cookie dough that you can roll out. When you put a little bit in your fingers you should be able to press and mold it together.
In this step, you have to work pretty quickly because it can dry out. Layout a large piece of wax or parchment paper, dust it generously with powdered sugar. Place the potato mixture on the paper, add a generous amount of powdered sugar on top.
Roll it out to ¼ inch thick. Try to get this into a rectangle shape as well as you can.
Smear the peanut butter all over the top leaving a half-inch border.
Tightly roll into a long log. Place on a piece of plastic wrap and roll the log up and tuck the ends under. Place in the refrigerator for 10 minutes to set.
Unwrap on a cutting board and cut into ¼-½ inch slices to serve.
Do not use instant potatoes for this recipe, they are too watery.
You can prepare your potato any way you like, we microwave ours.
Creamy peanut butter is best but crunchy can be used as long as when it is spread you don't tear or poke holes in the potato mixture.
If you don't have salted butter unsalted can be used, just add a bit of salt along with it.