1. In electric stand mixer bowl, drop gelatin and ½ cup of cold water. Let it stand for 15 minutes.
2. Grease an 8x8-inch cake pan with butter. Lay parchment paper and dust generously with icing sugar. Set it aside.
3. Place a heavy saucepan on medium low heat and add remaining water, sugar, corn syrup, salt, and vanilla. Stir and cook until sugar dissolves.
4. Turn the heat to high and let it boil, without stirring until the temperature reaches 240 degree F when measured with a candy thermometer.
5. Meanwhile, add egg whites into the gelatin mixture and whisk on high setting until stiff peaks form.
6. Reduce the speed to low and gradually pour hot sugar mixture into the mixer.
7. Turn the blender back to high setting and whisk for 15 minutes.
8. Pour mixture in cake pan and spread evenly with a greased spatula.
9. Sprinkle more sugar on top and let it rest, uncovered overnight under room temperature.