Prep a 8×8 or 9×9 glass dish with butter and set to the side
Over medium heat, add sugar, corn syrup, and water. Stir til just combined and do not touch to avoid sugar crystals from forming.
While the sugar mixture is heating, add gelatin and water to mixing bowl and stir til combined and let bloom.
With a candy or instant thermometer, test the mixture and when it reaches 240-245F(softball stage), remove from heat.
On low, add the sugar mixture to the bloomed gelatin. Once all added, turn the mixture to high for 3 minutes
Add vanilla, and mix for 2 more minutes, until light and fluffy
Add to buttered dish and smooth out. Cover for 5-6 hours
When ready, mix 1 tbsp cornstarch or arrowroot powder and 1 tbsp powdered sugar in a separate bowl. Sift over the top and flip over onto a cutting board. Cut into desired size. Coat each marshmallow in powder. Enjoy!