01 - Lightly grease a 9x9-inch baking pan. Mix confectioners sugar and cornstarch together, then dust the pan with the mixture. Reserve remaining mixture for later use.
02 - Combine gelatin and 1/2 cup cold water in the bowl of a stand mixer. Let stand for 5 minutes to bloom while preparing the syrup.
03 - In a medium saucepan, combine granulated sugar, corn syrup, 1/2 cup water, and salt. Stir over medium heat until sugar dissolves. Increase heat to high and bring to a boil without stirring. Cook until mixture reaches 240°F on a candy thermometer.
04 - With mixer on low speed, carefully pour hot syrup into bloomed gelatin. Gradually increase speed to high and beat for 10–12 minutes until mixture is thick, glossy, and tripled in volume. Add vanilla extract during the final minute of mixing.
05 - Quickly spread mixture evenly into prepared pan using a greased spatula. Smooth the top and dust with additional sugar-cornstarch mixture. Let set uncovered at room temperature for at least 4 hours or overnight.
06 - Turn marshmallow slab onto a cutting board. Cut into 1-inch squares with a sharp knife or pizza cutter dusted in the sugar-cornstarch mixture. Toss each piece in remaining mixture to prevent sticking.