Prepare Your Pan: Grease a 9x13-inch baking dish and dust it with a mixture of powdered sugar and cornstarch. This prevents sticking and makes it easier to remove the marshmallows later. Set the remaining sugar and cornstarch aside for later.
Bloom the Gelatin: In the bowl of a stand mixer or using a hand mixer, combine gelatin with ½ cup of cold water. Let it sit while you prepare the syrup.
Make the Sugar Syrup: In a saucepan over medium heat, combine the remaining ½ cup of water, granulated sugar, corn syrup, and salt. Stir gently until the sugar dissolves, then stop stirring and let the mixture reach 240°F (soft-ball stage) using a candy thermometer.
Whip the Marshmallow Base:CAREFULLY pour the hot syrup into the bloomed gelatin while mixing on low speed. Once combined, increase to high speed and whip for about 10-12 minutes until the mixture becomes thick, glossy, and triples in volume. This may take a little longer if using a hand mixer. Add vanilla extract in the last minute of mixing.
Spread and Set: Pour the fluffy marshmallow mixture into the prepared pan, smoothing the top with a spatula. Let it sit uncovered for at least 6 hours (or overnight) until firm.
Cut and Coat: Dust a cutting board with powdered sugar and cornstarch. Flip the marshmallows out of the pan and cut into squares using a sharp knife or pizza cutter. Toss each piece in the powdered sugar mixture to prevent sticking.