01 - Line a 9x9-inch pan with parchment paper. Dust generously with half of the confectioners' sugar and cornstarch mixture, coating the bottom and sides completely.
02 - Combine gelatin powder and 1/2 cup cold water in the bowl of a stand mixer. Let stand for at least 10 minutes until the gelatin absorbs the water and becomes firm.
03 - Combine granulated sugar, corn syrup, 1/4 cup water, and salt in a saucepan. Stir gently to moisten sugar.
04 - Heat over medium heat until sugar dissolves completely. Increase heat to bring to a rolling boil without stirring. Attach a candy thermometer and cook until reaching 240°F, approximately 10-12 minutes.
05 - With mixer on low speed, carefully pour the hot sugar syrup into the bloomed gelatin. Gradually increase speed to high and whip for 8-10 minutes until the mixture is white, thick, and tripled in volume.
06 - Add vanilla extract and mix for an additional 30 seconds until fully incorporated.
07 - Working quickly before the mixture sets, pour the marshmallow into the prepared pan. Use a damp spatula to smooth the top evenly.
08 - Dust the surface generously with the remaining confectioners' sugar and cornstarch mixture. Leave uncovered at room temperature for at least 4 hours, preferably overnight, until completely set.
09 - Turn the marshmallow slab onto a cutting board. Cut into squares using a sharp knife dusted with powdered sugar. Toss each piece in the sugar-cornstarch mixture to coat all sides.