Combine mashed potatoes, milk, vanilla extract, and salt in a bowl. Stir confectioners’ sugar into potato mixture until a dough consistency is reached.
Refrigerate dough until chilled, about 1 hour. Sprinkle confectioners’ sugar on a cutting board or waxed paper.
Roll dough into a large rectangular shape on prepared surface. Spread enough peanut butter on top layer of dough to cover.
Roll dough into a jelly roll shape; refrigerate roll for 1 hour.
Slice dough into pinwheels to serve.