Mix sifted sugar into potatoes a little at a time by hand. (Don't panic when potatoes liquify as the first sugar is added. This is what it's supposed to do. Just keep adding sugar until it is pastry consistency.) Sprinkle wax paper with additional powdered sugar. Take baseball-sized ball of mixture and roll out like pastry. Spread with peanut butter and roll like a jelly roll. Wrap rolls in plastic wrap; chill and slice.