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 during cutting.
02 - Combine gelatin and 1/2 cup cold water in the bowl of a stand mixer. Let it bloom and soften while you prepare the sugar syrup.
03 - In a medium saucepan, combine granulated sugar, corn syrup, 1/2 cup water, and salt. Stir over medium heat until sugar completely 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 the 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 surface. Dust with additional 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 using a sharp knife or pizza cutter dusted with the sugar and cornstarch mixture. Toss each piece in remaining mixture to prevent sticking.