01 - Line a 9x9-inch pan with parchment paper. Dust generously with half of the confectioners' sugar and cornstarch mixture, tapping to coat evenly.
02 - In the bowl of a stand mixer, combine gelatin powder and 1/2 cup cold water. Let stand for at least 10 minutes until gelatin absorbs the liquid and becomes firm.
03 - In a medium saucepan, combine granulated sugar, corn syrup, 1/4 cup water, and salt. Stir gently to moisten all ingredients.
04 - Heat over medium heat, stirring occasionally until sugar dissolves completely. Increase heat to medium-high and bring to a rolling boil without stirring. Attach a candy thermometer and cook until mixture reaches 240°F (soft ball stage), about 10-12 minutes.
05 - With the stand mixer on low speed, carefully pour the hot sugar syrup into the bloomed gelatin in a thin, steady stream down the side of the bowl.
06 - Gradually increase mixer speed to high. Whip for 8-10 minutes until mixture is thick, glossy white, and has tripled in volume. The bowl should feel warm to the touch.
07 - Add vanilla extract and mix on medium speed for 30 seconds until fully incorporated.
08 - Working quickly before the mixture sets, pour marshmallow into the prepared pan. Use a spatula sprayed with cooking spray or dipped in water to smooth the top evenly.
09 - Sprinkle the remaining confectioners' sugar and cornstarch mixture evenly over the top. Let sit uncovered at room temperature for at least 4 hours, preferably overnight, until firm and set.
10 - Turn the marshmallow slab onto a cutting board. Using a sharp knife dusted with powdered sugar, cut into 24 squares. Toss cut marshmallows in remaining sugar-cornstarch mixture to coat all sides.
11 - Store marshmallows in an airtight container at room temperature for up to 1 week. Keep layers separated with parchment paper to prevent sticking.