Prepare the pan
Lightly grease a 9x13-inch baking dish and line it with parchment paper.
In a small bowl, mix powdered sugar and cornstarch. Dust the parchment with this mixture.
Bloom the gelatin
In the bowl of a stand mixer, add gelatin and 1/2 cup cold water. Let sit for 10 minutes to bloom.
Make the syrup
In a medium saucepan over medium-high heat, combine granulated sugar, corn syrup, salt, and remaining 1/2 cup water.
Stir until sugar dissolves, then stop stirring. Attach a candy thermometer and cook until mixture reaches 240°F (soft-ball stage), about 8–10 minutes.
Combine gelatin and syrup
Turn the stand mixer to low speed and slowly pour the hot syrup into the bloomed gelatin.
Once all syrup is added, increase speed to high and whip for 10–12 minutes, until the mixture is thick, glossy, and tripled in volume.
Add vanilla extract in the last minute of mixing.
Spread and set
Immediately pour marshmallow mixture into the prepared pan. Smooth the top with a greased spatula.
Dust the top with more powdered sugar-cornstarch mixture.
Let sit uncovered at room temperature for at least 6 hours (or overnight).
Cut and coat
Lift marshmallow slab from pan and place on a cutting board.
Use a sharp knife or pizza cutter (dusted with powdered sugar) to cut into squares.
Toss each marshmallow in remaining powdered sugar-cornstarch mixture to prevent sticking.