Ingredients

• 1 cup smooth plain mashed potatoes
• Kosher salt
• 6 to 8 cups confectioners' sugar (about 2 boxes), plus more for dusting
• 1 cup crunchy peanut butter, at room temperature
• 1/4 cup umeboshi paste (available at health-food stores) or raspberry jam

Instructions

1. Combine the mashed potatoes and 1/4 teaspoon salt in a large bowl. Add 4 cups confectioners' sugar and stir with a wooden spoon. Stir in the remaining 2 to 4 cups confectioners' sugar, 1 cup at a time, until the dough is stiff and dry. (The amount of sugar will depend on how starchy your mashed potatoes are.) Knead with your hands to bring the dough together.
2. Dust a large sheet of parchment paper with confectioners' sugar. Transfer the dough to the parchment and roll into a 12-by-14-inch rectangle, about 1/4 inch thick, dusting with confectioners' sugar if the dough gets sticky. (Refrigerate for a few minutes if the dough gets too soft to roll.) Spread the peanut butter evenly over the dough. Spread a long narrow strip of umeboshi paste on top of the peanut butter, about 3 inches in from one of the long ends. Refrigerate 30 minutes.
3. Starting with one of the long ends, roll up the dough like a jelly roll, using the parchment to help you. Turn seam-side down and slice into thin pieces.
4. Photograph by Kang Kim