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 and 1/2 cup cold water. Let sit for at least 10 minutes to bloom.
03 - In a saucepan, combine granulated sugar, corn syrup, 1/4 cup water, and salt. Stir gently over medium heat until sugar dissolves.
04 - Increase heat 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 hot syrup into bloomed gelatin. Gradually increase to high speed and whip for 8-10 minutes until mixture is white, thick, and tripled in volume.
06 - Add vanilla extract and mix for another 30 seconds until fully incorporated.
07 - Quickly pour marshmallow mixture into prepared pan. Smooth top with a damp spatula. Dust with remaining sugar-cornstarch mixture. Let sit uncovered at room temperature for at least 4 hours or overnight until set.
08 - Turn slab onto cutting board. Cut into squares using a sharp knife dusted with powdered sugar. Toss marshmallows to coat all sides in sugar-cornstarch mixture.
09 - Store leftovers in an airtight container for up to 1 week.