01 - Line a 9x9-inch baking pan with parchment paper and lightly dust with a mixture of confectioners sugar and cornstarch.
02 - Sprinkle gelatin over 1/2 cup cold water in the bowl of a stand mixer and let bloom for 10 minutes.
03 - In a medium saucepan, combine granulated sugar, corn syrup, and 1/2 cup water; stir gently to combine.
04 - Place the saucepan over medium-high heat and bring to a boil without stirring; cook syrup to 240°F using a candy thermometer.
05 - With the mixer on low speed, slowly pour hot syrup into gelatin mixture in a thin stream.
06 - Increase mixer speed to high and whip for 10 to 12 minutes until the mixture is thick, white, and tripled in volume.
07 - Add vanilla bean seeds or extract and fine sea salt; mix for 1 additional minute.
08 - Immediately transfer the marshmallow mixture to the prepared pan and smooth the top with a greased spatula.
09 - Dust the surface with confectioners sugar and cornstarch mixture; let set uncovered at room temperature for at least 4 hours or overnight.
10 - Turn marshmallow slab onto a cutting board, peel off parchment, cut into squares with a greased knife, and toss each piece in confectioners sugar and cornstarch to prevent sticking.