01 - Line a 9x9-inch pan with parchment paper. Combine confectioners' sugar and cornstarch, then dust the pan generously with half the mixture.
02 - In a stand mixer bowl, combine gelatin powder with 1/2 cup cold water. Let stand for at least 10 minutes until gelatin absorbs the liquid 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 completely dissolves.
04 - Increase heat to high and bring mixture to a boil without stirring. Attach a candy thermometer and cook until reaching 240°F (soft ball stage), approximately 10–12 minutes.
05 - With mixer running 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 for another 30 seconds until fully incorporated.
07 - Working quickly, pour marshmallow mixture into prepared pan. Smooth top with a damp spatula. Dust surface with remaining sugar-cornstarch mixture. Let sit uncovered at room temperature for at least 4 hours or preferably overnight until completely set.
08 - Turn marshmallow slab onto a cutting board. Cut into squares using a sharp knife dusted with powdered sugar. Toss pieces to coat all sides in the sugar-cornstarch mixture.
09 - Store marshmallows in an airtight container at room temperature for up to 1 week.