Dust a 9x9 inch square dish generously with confectioners' sugar.
In a small saucepan over medium-high heat, stir together white sugar, corn syrup and ¾ cup water. Cook until it reaches 250ºF to 265ºF, or until a small amount of syrup dropped into cold water forms a rigid ball.
While the syrup is heating, place the remaining ½ cup water in a small saucepan and sprinkle the gelatin over the surface. Let it rest for a few minutes. Place the saucepan over low heat, and whisk until the gelatin has dissolved completely. Keep the gelatin mixture in a warm place until the syrup has come to temperature.
Remove the syrup from the heat and whisk in the gelatin mixture.
In a separate large bowl, whip the egg whites with a hand mixer until soft peaks form. Continue to beat while your pour the syrup mixture into egg whites in a thin stream, until the egg whites are very stiff. Stir in the vanilla.
Spread the mixture evenly into your prepared pan. Let rest for 8 hours or overnight before cutting.
Recipe by Go Dairy Free at https://www.godairyfree.org/recipes/homemade-marshmallows-dairy-free