In a small bowl, combine the cold water with the powdered gelatin; set aside.
To a saucepan, add the sugar, corn syrup, water and salt. Stir and heat over medium-high heat until the mixture reaches 235℉ on a candy thermometer (should take 6 to 8 minutes).
In a large bowl, combine the corn syrup mixture, gelatin mixture and the seeds from the 2 vanilla beans. Use a mixer to beat until thick- about 6 minutes.
In a small bowl, combine the powdered sugar and cornstarch. Sprinkle half of this mixture in an 8-inch square pan. Spread the marshmallow mixture in the pan with a buttered spatula, and then sprinkle the remaining cornstarch mixture on top. Let the marshmallows dry overnight. Then slice into squares (5x5 to make 25 marshmallows). Enjoy munching and in hot cocoa too!