For the "butter" frosting: Line the inside of a cover of a butter dish with plastic wrap. Fill the cover completely with yellow frosting and freeze until hard. Lift the "butter" out of the cover using the plastic wrap. Cut into squares.;
For the potato ice cream: Have a sheet pan covered with plastic wrap. On 1/2 of the pan, place the pint of ice cream down on its side. Fold the plastic over the ice cream and make a seal.
Using your hands, quickly form the ice cream into a potato shape. Then, create a wedge across the length of the potato. Tightly twist the ends of the plastic wrap and make knots at each end, like a mozzarella ball.
Place the ice cream in the freezer until it hardens, about 20 to 30 minutes.
Remove the ice cream from the freezer, unwrap and dust all over with cocoa powder. Fill the wedge with whipped topping and place a pat of "butter" frosting on top. Sprinkle with pistachios and serve immediately.