Lightly grease a 9x13 inch baking dish with non-stick spray, then line it with parchment paper, allowing an overhang on the sides. Grease the parchment paper again and set aside.

In the bowl of a stand mixer, sprinkle the gelatin over 1/2 cup of cold water and let it sit for 5-10 minutes to bloom.

In a large heavy-bottomed saucepan, combine the granulated sugar, corn syrup, 1/2 cup water, and salt. Cook over medium-high heat, stirring until the sugar dissolves.

Insert a candy thermometer and continue to cook without stirring until the mixture reaches 240°F (115°C).

Carefully remove the hot syrup from the heat and immediately pour it in a steady stream into the bloomed gelatin in the stand mixer, with the mixer on low speed.

Gradually increase the mixer speed to high and whip for 10-15 minutes, or until the mixture is very thick, white, and nearly tripled in volume. Beat in the vanilla extract during the last minute of whipping.

Pour the marshmallow mixture into the prepared baking dish, using an oiled spatula to spread it evenly. Let it sit uncovered at room temperature for at least 4-6 hours, or preferably overnight, to set completely.

In a small bowl, whisk together the powdered sugar and cornstarch for the dusting powder.

Once set, invert the marshmallow onto a cutting board dusted with the powder. Remove the parchment paper, dust the top generously, and use an oiled knife or pizza cutter to cut into squares.

Toss each marshmallow piece in the remaining dusting powder to prevent sticking.