01 - Lightly grease a 9x9-inch baking pan and dust with a mixture of confectioners sugar and cornstarch, reserving the 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 to allow gelatin to bloom.
03 - Combine granulated sugar, corn syrup, 1/2 cup water, and salt in a medium saucepan. Stir over medium heat until sugar dissolves, then increase heat and bring to a boil without stirring. Heat 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 to 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. Dust top 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 with the sugar and cornstarch mixture. Toss each piece in remaining mixture to prevent sticking.