Combine powdered sugar and cornstarch together in a medium bowl.
Lightly spray a 9x13-inch baking pan with cooking spray and dust with half of the powdered sugar mixture, making sure to coat the bottom and sides of the pan fully. Transfer the excess coating back to the bowl with the half that wasn't used.
In a medium saucepan, combine ½ cup water, sugar, corn syrup, and salt and cook covered over medium-high heat for 3 to 4 minutes until the edges of the mixture start to simmer. (DO NOT STIR)
Uncover and clip a candy thermometer to the side of the pan a continue to cook until the temperature reaches 240°F, (DO NOT STIR), about 7 to 8 minutes. Once the mixture reaches this temperature, immediately remove the pan from heat.
Stir the gelatin and ½ cup of water in the bowl of a stand mixer and let sit so the gelatin can bloom for about 5 minutes.
Turn the stand mixer on low speed (stir setting); while running, slowly pour the sugar mixture down the side of the bowl into the gelatin mixture.
Once you have added all the sugar syrup, increase speed to high (10 speed) and whip until the mixture becomes very thick, fluffy, and glossy, about 6 to 7 minutes. Add the vanilla and whip for 30 additional seconds.
Spread marshmallow cream out in the pan so it’s relatively level. Sprinkle with more powdered sugar mixture. Let sit uncovered for at least 4 hours, preferably overnight.
Once set, turn marshmallow out onto a cutting board and slice it into 1-inch squares with a pizza cutter, large knife, or unflavored dental floss dusted in the powdered sugar mixture.
Once cut, dust every side of each marshmallow with powdered sugar mixture, make more mixture if needed. Ten toss the marshmallows in a colander to remove any excess mixture.
Store in an airtight container for up to 3 weeks!