01 - Lightly grease a 9x9-inch baking pan and dust with a mixture of confectioners sugar and cornstarch. Reserve remaining mixture for later use.
02 - Combine gelatin and 1/2 cup cold water in the bowl of a stand mixer. Let stand 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 and bring to a boil without stirring. Heat to 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 thick, glossy, and tripled in volume. Add vanilla during the final minute of mixing.
05 - Quickly spread mixture evenly into prepared pan using a greased spatula. Smooth top. Dust with more confectioners sugar and cornstarch mixture. Let set, uncovered, at room temperature for at least 4 hours, preferably overnight.
06 - Turn marshmallow slab onto a cutting board. Cut into 1-inch squares with a sharp knife or pizza cutter dusted with the sugar and cornstarch mixture. Toss each piece in remaining mixture to prevent sticking.