01 - Line a 9x9-inch pan with parchment paper. Dust generously with half of the confectioners' sugar and cornstarch mixture, ensuring even coverage on bottom and sides.
02 - In the bowl of a stand mixer, combine gelatin powder with 1/2 cup cold water. Let stand for at least 10 minutes until the gelatin absorbs the liquid and becomes firm.
03 - In a medium saucepan, combine granulated sugar, corn syrup, 1/4 cup water, and salt. Stir gently just to moisten the sugar.
04 - Heat over medium heat, stirring occasionally until sugar completely dissolves. Increase heat to medium-high and bring to a rolling boil without stirring. Cook until mixture reaches 240°F on a candy thermometer, approximately 10-12 minutes.
05 - With stand mixer on low speed, carefully pour hot syrup into bloomed gelatin. Gradually increase speed to high and whip for 8-10 minutes until mixture is white, thick, and tripled in volume.
06 - Add vanilla extract and mix on medium speed for 30 seconds until fully incorporated.
07 - Working quickly before mixture sets, pour marshmallow into prepared pan. Use a damp spatula to smooth the top evenly. Dust surface with remaining confectioners' sugar and cornstarch mixture. Let stand uncovered at room temperature for at least 4 hours, preferably overnight.
08 - Turn set marshmallow slab onto a cutting board. Using a sharp knife dusted with powdered sugar, cut into 24 squares. Toss pieces in remaining sugar-cornstarch mixture to coat all sides completely.