Prepare the Pan: Line an 8×8-inch baking dish with parchment paper. Lightly grease the paper and dust it generously with powdered sugar to prevent sticking.
Bloom the Gelatin: In the bowl of a stand mixer, add ½ cup of cold water. Sprinkle the gelatin over the water and let it sit for 5–10 minutes to bloom.
Cook the Syrup: In a medium saucepan, combine the remaining ½ cup of cold water, granulated sugar, corn syrup, and salt. Stir gently to combine. Attach a candy thermometer to the pan and cook over medium heat without stirring until the mixture reaches 240°F (soft-ball stage). Combine Syrup with Gelatin: Turn the mixer on low speed and carefully pour the hot syrup into the bloomed gelatin. Be cautious as the syrup is extremely hot.
Whip the Mixture: Increase the mixer speed to high and whip for about 10–12 minutes, or until the mixture becomes thick, fluffy, and lukewarm. Add the vanilla extract in the final minute of whipping.
Transfer to Pan: Using a greased spatula, spread the marshmallow mixture evenly into the prepared pan. Smooth the top as much as possible. Dust the surface generously with powdered sugar.
Set the Marshmallows: Let the marshmallows sit at room temperature for at least 6 hours, or overnight, until fully set.
Cut and Coat: Dust a cutting surface with powdered sugar. Turn the marshmallows out onto the surface and peel away the parchment paper. Use a sharp, greased knife or pizza cutter to slice the marshmallows into squares. Toss each piece in powdered sugar to coat all sides.