01 - Lightly grease a 9x9-inch baking pan and dust with a mixture of confectioners sugar and cornstarch. Reserve remaining mixture for coating cut marshmallows.
02 - Combine gelatin and 1/2 cup cold water in the bowl of a stand mixer. Let stand 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 boil without stirring until mixture reaches 240°F on a candy thermometer.
04 - With mixer on low speed, carefully pour hot syrup into gelatin mixture. Gradually increase to high speed and beat for 10-12 minutes until thick, glossy, and tripled in volume. Add vanilla during the final minute of mixing.
05 - Quickly spread marshmallow mixture evenly into prepared pan using a greased spatula. Smooth the top and dust with additional 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 in the sugar-cornstarch mixture. Toss each piece in remaining mixture to prevent sticking.