Lightly dust a clean surface with the icing sugar mixture. Unmould the marshmallow and remove the plastic wrap. Using a knife with a lightly moistened blade, cut the marshmallow into 4-cm (1 1/2-inch) squares. Roll each square in the icing sugar mixture, shaking off any excess. Place the marshmallows on a cookie sheet and let warm to room temperature. If necessary, roll a second time in the icing sugar mixture.