01 - Line a 9x9-inch baking pan with parchment paper, lightly grease and dust with a mixture of confectioners sugar and cornstarch.
02 - Sprinkle gelatin over ½ cup cold water in the bowl of a stand mixer and let it bloom while preparing the sugar syrup.
03 - Combine granulated sugar, corn syrup, ½ cup water, and salt in a medium saucepan. Stir over medium heat until sugar dissolves. Increase heat and boil without stirring until the syrup reaches 240°F (115°C).
04 - With the mixer on low, slowly pour hot syrup into the gelatin mixture in a thin stream.
05 - Increase mixer speed to high and beat for 10 to 12 minutes until thick, glossy, and tripled in volume. Add vanilla seeds or extract during the last minute.
06 - Quickly pour the mixture into the prepared pan and smooth the surface with a greased spatula.
07 - Dust the top with the remaining confectioners sugar and cornstarch mixture. Let set uncovered at room temperature for at least 3 hours until firm.
08 - Invert marshmallow slab onto a cutting board, remove parchment, cut into 1-inch squares with a greased sharp knife or pizza wheel, and toss pieces in remaining sugar/starch mixture to prevent sticking.