Oil the bottom of a 9x13x2 inch metal pan and dust sides and bottom with confectioner's sugar. In a large mixing bowl add cold water and sprinkle gelatin over the water. Let it stand and soften. Meanwhile, in a heavy saucepan
, cook corn syrup, hot water, salt and granulated sugar over low heat, stirring with a wooden spoon. Increase heat to medium-high and boil, without stirring, until a candy thermometer reaches 240F, or for about 12 minutes. Remove from heat and pour sugar mixture over gelatin mixture, stirring until dissolved. Use a handheld or standing mixer to beat the mixture until thick, white, and tripled in volume, about 10 minutes with a handheld mixer or 6 minutes with a standing mixer. In a large bowl, beat eggwhites until stiff peaks form. Beat vanilla and whites into the sugar mixture until combined. Pour mixture into pan, spreading evenly. Chill mixture uncovered for at least 3 hours or up to 1 day. Run a knife around edges to loosen the marshmallow and invert pan over a cutting board to release. Cut marshmallow into strips, and then across, making 1 inch cubes. Put remaining confectioner's sugar into a large bowl and toss in marshmallow cubes, working in batches. Toss to completely coat. Keep marshmallows in an airtight container at room temperature for up to 1 week. Makes 96 marshmallows.