Spray the bottom of a 13 x 9-inch baking pan with nonstick cooking spray. Combine the cornstarch and powdered sugar in a small bowl. Sprinkle some of the cornstarch mixture over the bottom of the baking pan, shaking to evenly coat. Shake off any excess lumps or powder and set both pan and cornstarch mixture aside for later use. Pour the gelatin and 1/2-cup of the water into the bowl of an electric mixer fitted with a whisk. Immediately proceed to the next step. In a large heavy saucepan, combine the granulated sugar and corn syrup with the remaining 1/2-cup of water over medium heat. Stir over medium heat until sugar and corn syrup dissolve. Increase heat to high and attach a candy thermometer. Let the mixture cook (do not stir) until the temperature reaches 240ºF. Remove from heat and, with the mixer set on low speed, add the sugar solution to the gelatin mixture in a slow, steady stream. Once combined, whip the mixture on high for 10 to 12 minutes until the batter has thickened (batter will double in size), is milky white, and forms medium peaks. Stir in the vanilla extract. Meanwhile, heat the coconut over medium heat in a dry nonstick skillet. Once the coconut starts to brown, stir constantly to avoid burning. When it is evenly golden, remove from heat and allow to cool. Pour marshmallow batter into prepared pan and spread with a rubber spatula to form one even layer. Sprinkle toasted coconut generously over the top, gently pressing down to adhere. Leave marshmallows uncovered and at room temperature to dry overnight. Cut into 12 squares. Coat sides of portioned marshmallows in remaining cornstarch mixture and store in an airtight container. Fresh marshmallows will keep tightly covered for two days at room temperature, or up to one week if refrigerated.
