1. Stir the icing sugar with the cornstarch; sift enough of the mixture on to a 9 x 13-inch (3 L) baking pan to make a generous bed of powder; set aside. Reserve the remaining sugar mixture.
2. Soak the gelatin in the cold water for 1 minute. Pour the boiling water over the gelatin mixture and whisk until smooth; set aside.
3. Position a candy thermometer on the side of a large, heavy saucepan so the tip will be submerged in the mixture but not touch the bottom of the pan. Stir the granulated sugar, corn syrup and room temperature water in the saucepan; bring to a boil. Cook, without stirring, for about 5 minutes or until the mixture reaches 265°F on the candy thermometer.
4. Meanwhile, beat the egg whites in a heatproof bowl until very stiff and glossy. Once the sugar mixture reaches temperature, remove the pan from the heat and with the mixer running, carefully and slowly drizzle the hot syrup into the egg whites. Add the gelatin mixture and jam; continue to beat for 15 minutes or until the mixture is very fluffy and is cool to the touch.
5. Spread the mixture in the prepared pan and smooth the top. Let stand, uncovered, for at least 6 hours but preferably overnight. Use a wet, thin knife to cut the marshmallows into 36 pieces; toss with the reserved icing sugar mixture to coat.
- Marshmallows can be prepared and stored in an airtight container for up to 1 week.
- Bring s'mores inside and top chocolate covered Graham crackers with a marshmallow and broil until golden.