2/3 cup water, divided
3 (1/4 oz.) envelopes unflavored gelatin
1 cup sugar
1 cup light corn syrup
Pinch of salt
1 tsp. vanilla
1 cup powdered sugar
Lightly spray the inside of an 8x8 pan with nonstick cooking spray. Generously coat with powdered sugar and set aside.
Pour 1/3 cup of the water into the bowl of a stand mixer. Sprinkle the gelatin over the water, and stir briefly to incorporate. Let stand for about 10 minutes, or until the gelatin has softened.
In a saucepan, off the heat, combine the remaining 1/3 cup of water and the granulated sugar, corn syrup and salt. Place the pan over medium-high heat. Cook the mixture without stirring until it reaches 240ºF. Brush down the sides of the pan with a pastry brush, dipped in water, to gently wipe away any sugar crystals.
With your mixer on low speed, very carefully add the hot syrup to the softened gelatin. Add the vanilla, and increase the speed to medium-high. The mixture will start out clear, but quickly turns very white.
Beat for about 8 minutes, or until the marshmallow gets very thick and sticky and starts to pull away from the sides of the bowl, in string-like pieces.
Spread the mixture into the prepared pan using a lightly oiled spatula. With damp hands, press the batter evenly into the corners of the pan. Just keep rewetting your hands to help with the stickiness. Set aside for at least 1 hour or until the mixture is firm and cool.
Sift the powdered sugar into a shallow bowl. Run a wet knife around the edge of the cooled pan to loosen the marshmallow. Remove the marshmallow from the pan. Cut into squares, wetting the knife as needed, to keep it from sticking. If you have a pizza cutter, I found that worked better than a knife. Cut the marshmallow into 36 squares to have marshmallows that are about 1 1/4-inch squares, but feel free to cut them into whatever size you like. Toss each marshmallow in the powdered sugar until completely coated.
Store marshmallows in a single layer, or in layers separated by wax paper. They will keep very well for quite a long time (a few weeks!) when stored airtight at moderate temperature.