1. Butter a 9-by-13-inch cake pan.
2. Sift the cornstarch with the confectioner’s sugar in a bowl. Use half of this mixture to coat the pan and reserve remaining half for after the marshmallow has set.
3. Soften the gelatin with 1/2 cup of the water in a small bowl.
4. In a medium saucepan, combine the remaining 1/3 cup water, the sugar, the salt, and the corn syrup and bring to a boil. When the temperature reaches 245 degrees F. on a candy thermometer, remove pan from the heat. Add the gelatin mixture and stir to dissolve.
5. Beat the egg whites in a stand mixer until soft peaks form. Beat in the hot syrup on medium speed in a thin stream. Increase speed to high and beat whites until cooled, about 10 minutes. Beat in the orange blossom water.
6. With a spatula spread the mixture in the prepared pan and let set for 3 hours.
7. Sift some of the reserved cornstarch mixture over the top. Run a paring knife around the sides and un-mould onto a cutting board (you may need to torch the bottom or dip it in a pan of hot water to un-mold the bottom).
8. Cut the marshmallow into big chunks. Sprinkle the remaining cornstarch mixture onto a parchment-lined sheet tray. Place the chunks on the paper, roll them around in the cornstarch to cover all sides, and let set for another 3 hours.