01 - Line a 9x9-inch pan with parchment paper and dust generously with half of the confectioners' sugar and cornstarch mixture.
02 - In the bowl of a stand mixer, combine gelatin and 1/2 cup cold water. Let bloom for at least 10 minutes.
03 - In a saucepan, combine granulated sugar, corn syrup, 1/4 cup water, and salt. Stir gently to combine.
04 - Heat over medium heat until sugar dissolves. Increase heat and bring to a boil without stirring. Using a candy thermometer, cook until 240°F, about 10–12 minutes.
05 - With the mixer on low speed, carefully pour the hot syrup into the bloomed gelatin. Gradually increase speed to high and whip for 8–10 minutes, until white, thick, and tripled in volume.
06 - Add vanilla and mix for another 30 seconds until fully incorporated.
07 - Working quickly, pour marshmallow mixture into prepared pan. Smooth top with a damp spatula.
08 - Dust the top with remaining confectioners' sugar and cornstarch mixture. Let sit uncovered at room temperature for a minimum of 4 hours, preferably overnight, until completely set.
09 - Turn marshmallow slab onto a cutting board. Cut into squares with a sharp knife dusted with powdered sugar. Toss marshmallows to coat all sides in the sugar-cornstarch mix.
10 - Store leftovers in an airtight container for up to 1 week.