01 - Lightly grease a 9x9-inch baking pan and line it with parchment paper; generously dust with a mixture of confectioners sugar and cornstarch.
02 - In the bowl of a stand mixer fitted with a whisk attachment, combine gelatin and 1/2 cup cold water; allow to bloom while preparing syrup.
03 - Combine granulated sugar, corn syrup, 1/2 cup water, and salt in a medium saucepan; stir over medium heat until sugar dissolves.
04 - Increase heat to high and bring mixture to a boil without stirring; cook until syrup reaches 240°F on a candy thermometer.
05 - With mixer on low speed, slowly pour hot syrup into bloomed gelatin in a thin, steady stream.
06 - Increase mixer speed to high and whip for 10 to 12 minutes until mixture is thick, glossy, and has tripled in volume.
07 - Beat in vanilla bean paste and vanilla extract until just combined.
08 - Quickly pour marshmallow mixture into prepared pan and smooth the surface with a greased spatula.
09 - Dust top with more confectioners sugar and cornstarch; let set uncovered at room temperature for 3 to 4 hours until firm.
10 - Invert marshmallow slab onto a cutting board; remove parchment and cut into squares using a knife dusted with the sugar-cornstarch mixture; toss cut pieces in remaining mixture to coat all sides.