Emily's Famous Marshmallows

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 3/4 cup water. Heat to 250 to 265 degrees F (121 to 129 degrees C), or until a small amount of syrup dropped into cold water forms a rigid ball.
While syrup is heating, place remaining water in a metal bowl and sprinkle gelatin over the surface. Place bowl over simmering water until gelatin has dissolved completely. Keep in a warm place until syrup has come to temperature. Remove syrup from heat and whisk gelatin mixture into hot syrup. Set aside.
In a separate bowl, whip egg whites to soft peaks. Continue to beat, pouring syrup mixture into egg whites in a thin stream, until the egg whites are very stiff. Stir in vanilla. Spread evenly in prepared pan and let rest 8 hours or overnight before cutting.