01 - Line a 9x9-inch pan with parchment paper. Mix confectioners' sugar and cornstarch together, then dust the pan generously with half of the mixture.
02 - In the bowl of a stand mixer, combine gelatin powder and 1/2 cup cold water. Let stand for at least 10 minutes until gelatin absorbs the water and becomes firm.
03 - In a saucepan, combine granulated sugar, corn syrup, 1/4 cup water, and salt. Stir gently to moisten all ingredients.
04 - Heat over medium heat until sugar completely dissolves. Increase heat to bring mixture to a boil without stirring. Attach a candy thermometer and cook until reaching 240°F, 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 before mixture sets, pour marshmallow into prepared pan. Use a damp spatula to smooth the top evenly.
08 - Sprinkle remaining sugar-cornstarch mixture over the top. Let sit uncovered at room temperature for at least 4 hours, preferably overnight, until completely set.
09 - Turn marshmallow slab onto a cutting board. Cut into squares using a sharp knife dusted with powdered sugar. Toss cut pieces in remaining sugar-cornstarch mixture to coat all sides.
10 - Store marshmallows in an airtight container for up to 1 week.