01 - Lightly grease a 9x9-inch baking pan and dust with a mixture of confectioners sugar and cornstarch. Reserve the remaining mixture for later use.
02 - Combine gelatin and 1/2 cup cold water in the bowl of a stand mixer. Let it bloom while you prepare 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, then 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 the top and dust with more confectioners sugar-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-cornstarch mixture. Toss each piece in remaining mixture to prevent sticking.