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 bloom while preparing the syrup.
03 - Combine granulated sugar, corn syrup, 1/2 cup water, and salt in a medium saucepan. 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 and dust with confectioners sugar and cornstarch mixture. Let set, uncovered, at room temperature for at least 4 hours or preferably overnight.
06 - Turn marshmallow slab onto a cutting board. Cut into 1-inch squares with a sharp knife or pizza cutter dusted in sugar and cornstarch mixture. Toss each piece in remaining mixture to prevent sticking.