- Line a 20-cm (8-inch) square dish with plastic wrap and oil lightly.
- In a large bowl, sift the sugar and cornstarch together. Set aside.
- In a saucepan, sprinkle the gelatin on the strawberry purée. Let soften for 5 minutes. Add the sugar and bring to a boil, stirring to dissolve the gelatin. Remove from the heat and transfer to a bowl. Let cool for 5 minutes.
- Set the bowl over a saucepan partially filled with cold water; the water should touch the bottom of the bowl. Beat with an electric mixer until the mixture has cooled to room temperature and forms stiff peaks, 5 to 7 minutes (or about 15 minutes if the mixture is not whipped over cold water). Spread evenly in the dish and refrigerate until thoroughly chilled, about 3 hours.
- 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.
- The marshmallows can be stored for several days in an airtight container at room temperature.