Step 1
Grease a 9x13-inch baking pan with cooking spray; dust with 1 tablespoon confectioners' sugar.
Step 2
Pour 1/2 cup lukewarm water into the bowl of a stand mixer fitted with the whisk attachment. Sprinkle 2 tablespoons plus 3/4 teaspoon gelatin on top; stir briefly to combine.
Step 3
Combine sugar corn syrup 1/2 cup water and salt in a large saucepan. Cook over medium-low heat stirring constantly until sugar dissolves about 5 minutes. Increase heat and cook stirring constantly until mixture registers 250 degrees F (121 degrees C) on a candy thermometer about 5 minutes. Remove from heat.
Step 4
Turn the mixer on low speed and while running pour the hot sugar syrup in a thin stream down the side of the bowl. Increase speed to high and beat until marshmallow mixture is lukewarm and triples in volume about 15 minutes. Mix vanilla extract.
Step 5
Pour marshmallow mixture into the prepared pan. Press into an even layer with wet hands. Dust with 2 tablespoons confectioners' sugar. Let stand at room temperature uncovered until set 8 hours to overnight.
Step 6
Cut marshmallow mixture into desired shapes and toss in remaining 2 tablespoons confectioners' sugar.