To make potato candy, roll out a dough of sugared mashed potatoes, cover it with peanut butter, roll the dough into a log, freeze it and slice it. Store leftover candy in the refrigerator for up to seven days.
Put 1/3 cup mashed potatoes into the bowl of an electric mixer and pour in 1 tablespoon milk. Add 1 teaspoon vanilla extract and 1/8 teaspoon salt. Beat the mixture for two minutes, and then add 6 cups powdered sugar one cup at a time. Add additional powdered sugar by tablespoons until a cohesive dough forms.
Lay a sheet of parchment paper over your work surface. Turn the dough out onto the paper, and dust it with powdered sugar. Roll the dough out to a 1/8-inch thickness, and trim the edges to form a rectangle that measures 10 inches by 12 inches. Sprinkle the surface with powdered sugar, and cover the dough with a fresh sheet of parchment.
Slide one hand beneath the bottom sheet of parchment, and flip the dough over. Peel away and discard the top parchment layer. Cover the surface of the dough with 1/3 cup smooth peanut butter, and spread it into a thin, even layer. Lift up one long edge of the dough, and roll the candy up to the other long edge. Set the roll on a fresh sheet of parchment paper, roll it up tightly, and freeze for one hour.
After freezing, unwrap the candy, and cut it into 1/4-inch slices. Serve immediately or refrigerate in a sealed container.