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Make sure your mashed potatoes are plain and cool: If you’re using leftover mashed potatoes, they should be plain—no butter, milk, salt, or pepper. If they are seasoned, start with fresh potatoes: boil, mash with just a splash of water if needed, and cool completely in the fridge. You want 1/4 cup of smooth, cold mashed potatoes.
Start the candy dough: In a medium mixing bowl, add the 1/4 cup cooled mashed potatoes. Stir in 1 cup of powdered sugar at a time, mixing well after each addition. At first it will look thin and odd, but keep going. Add sugar until a soft dough forms that you can pick up without it sticking badly to your fingers. This usually takes around 4–5 cups, depending on how moist the potatoes are.
Adjust the texture: The dough should feel like a soft sugar cookie dough—pliable but not wet. If it’s sticky, sprinkle in more powdered sugar, a couple of tablespoons at a time, until it firms up. If it gets too stiff and crumbly, you can very lightly dampen your hands with water and knead it just a few times to bring it back together.
Chill the dough briefly: Shape the dough into a flat rectangle, wrap it in plastic wrap, and refrigerate for about 20–30 minutes. This makes it easier to roll out without tearing, especially if your kitchen is warm.
Prepare your rolling surface: Lay a sheet of parchment paper or wax paper on a speckled granite or other clean countertop. Dust the paper and a rolling pin lightly with powdered sugar so the dough doesn’t stick. If you don’t have parchment, you can dust the counter directly and use a little extra sugar as you roll.
Roll out the dough: Place the chilled dough on the prepared surface. Roll it into a rough rectangle about 1/4 inch thick. Aim for roughly 8 x 10 inches, but don’t stress about perfect edges—this is a humble, homey candy. If the dough sticks, dust lightly with more powdered sugar on top and underneath.
Spread the peanut butter: Gently spread the 1/2 cup creamy peanut butter evenly over the rolled dough, leaving about a 1/2-inch border around the edges so it doesn’t squeeze out when you roll. An offset spatula or the back of a spoon works well for this.
Roll into a log: Starting from one of the long sides, use the parchment or your hands to carefully roll the dough into a tight log, just like you would roll cinnamon rolls. Take your time, lifting and rolling so the dough doesn’t crack. If it does crack a bit, just pinch and smooth it back together—no one will notice once it’s sliced.
Wrap and chill: Once you have a log, dust your hands with a little powdered sugar and gently smooth the outside. Wrap the log snugly in the parchment or plastic wrap, twisting the ends like a candy wrapper. Place it in the refrigerator for at least 1–2 hours, or until very firm. This helps the pinwheels hold their shape when you slice them.
Slice the candy: When the log is firm, unwrap it and place it on a cutting board. Using a sharp knife, slice the log into 1/2-inch thick pieces. Wipe the knife clean with a damp cloth as needed to keep the slices neat. You’ll see the white candy with a brown peanut butter swirl in the center.
Arrange for serving: Set the slices cut-side up in a glass rectangular serving dish so the swirl shows, just like those old family picnic platters. If the candy softens while you’re arranging it, you can pop the dish back into the fridge for 10–15 minutes to firm up again before serving.
Store: Keep any leftovers covered in the refrigerator for up to 5 days. For a firmer, less sticky texture at a picnic, serve them straight from the fridge and don’t leave them sitting out in the sun. They can also be frozen in a single layer, then transferred to a container with parchment between layers for up to 1 month.
