Stir together 1/2 cup of lukewarm water and gelatin in the bowl of an electric mixer and allow to stand while making the syrup.
Add sugar, corn syrup, salt, and remaining 1/2 cup of lukewarm water in a medium saucepan over medium-high heat. Cook the mixture until the sugar has completely dissolved and the syrup reaches the softball stage, 240º F. Remove from heat.
Fit the mixer with the whisk attachment and turn the mixer onto the lowest setting while slowly pouring the syrup mixture into the gelatin mixture. Stop the mixer and make sure that all of the gelatin mixture is loose from the bottom of the mixing bowl. Turn the mixer on high speed and whip for 15 minutes. While the marshmallow mixture is whipping, lightly coat an 8 x 12 glass or non-metallic dish with coconut oil or shortening and dust with confectioner's sugar to make sure well-coated. Coat a rubber scraper with coconut oil or shortening and set aside.
Once marshmallow mixture has whipped 15 minutes and is light and fluffy in volume, stir in the vanilla until it is well incorporated. Spread the marshmallow mixture into the prepared dish using the oiled rubber scraper.
Dust lightly with more confectioner's sugar and leave uncovered overnight to dry completely.
Pour out onto parchment paper and cut into large pieces with a knife. Dust cut edges with more confectioner's sugar. Store in an airtight container for up to three weeks.