Peel and chop a small russet potato. Boil in water until very tender, then drain and mash until perfectly smooth. Cool slightly.
In a large bowl, blend 1⁄2 cup warm mashed potato with 2 cups powdered sugar and 1 tsp vanilla extract. Mix in more powdered sugar, 1 cup at a time, until a thick dough forms.
On a powdered-sugar-dusted surface, roll dough into a rectangle, 1⁄4 inch thick. Trim edges.
Spread peanut butter or Nutella evenly over dough. Gently roll up jelly-roll style. Wrap in wax paper and chill 1 hour.
Slice into 3⁄4 inch pieces. Store leftover rolls in an airtight container for up to 1 week.