01 - Line a 9x9-inch pan with parchment paper. Mix confectioners' sugar and cornstarch together, then dust the pan generously with half the mixture.
02 - In the bowl of a stand mixer, combine gelatin powder with 1/2 cup cold water. Let sit for at least 10 minutes until the gelatin absorbs the water and becomes firm.
03 - In a saucepan, combine granulated sugar, corn syrup, 1/4 cup water, and salt. Stir gently over medium heat until sugar dissolves completely.
04 - Increase heat to high and bring to a 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 mixture becomes white, thick, and triples in volume.
06 - Add vanilla extract and mix for another 30 seconds until fully incorporated.
07 - Working quickly before mixture sets, pour into prepared pan. Use a damp spatula to smooth the top evenly. Dust with remaining sugar-cornstarch mixture.
08 - Let sit uncovered at room temperature for minimum 4 hours, preferably overnight, until completely set and firm to the touch.
09 - Turn slab onto cutting board. Cut into squares using a sharp knife dusted with powdered sugar. Toss pieces in remaining sugar-cornstarch mixture to coat all sides.
10 - Keep stored in an airtight container at room temperature for up to 1 week.