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1
Combine the mashed potatoes and 1/4 teaspoon salt in a large bowl.
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2
Add 4 cups confectioners' sugar and stir with a wooden spoon.
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3
Stir in the remaining 2 to 4 cups confectioners' sugar, 1 cup at a time, until the dough is stiff and dry.
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4
(The amount of sugar will depend on how starchy your mashed potatoes are.)
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5
Knead with your hands to bring the dough together.
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6
Dust a large sheet of parchment paper with confectioners' sugar.
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7
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.
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8
(Refrigerate for a few minutes if the dough gets too soft to roll.)
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9
Spread the peanut butter evenly over the dough.
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10
Spread a long narrow strip of umeboshi paste on top of the peanut butter, about 3 inches in from one of the long ends.
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11
Refrigerate 30 minutes.
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12
Starting with one of the long ends, roll up the dough like a jelly roll, using the parchment to help you.
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13
Turn seam-side down and slice into thin pieces.
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14
Photograph by Kang Kim
