1. Lightly grease a 9×13 inch baking dish with non-stick spray and dust generously with a 50/50 mixture of powdered sugar and cornstarch.

2. In the bowl of a stand mixer, combine egg whites and half of the cold water (1/4 cup).

3. In a small bowl, sprinkle gelatin over the remaining 1/4 cup cold water and let it sit for 5 minutes to bloom.

4. In a medium saucepan, combine granulated sugar, corn syrup, and salt. Cook over medium-high heat, stirring constantly, until the sugar dissolves and the mixture reaches 240°F (soft-ball stage) on a candy thermometer.

5. Remove from heat and immediately stir in the bloomed gelatin until dissolved.

6. With the mixer on low speed, slowly pour the hot syrup mixture into the egg white mixture. Increase speed to high and beat for 10-12 minutes, or until the mixture is very thick, white, and holds stiff peaks. Beat in the vanilla extract.

7. Pour the marshmallow mixture into the prepared baking dish, spreading evenly with an oiled spatula. Sift the remaining powdered sugar and cornstarch mixture generously over the top.

8. Let sit uncovered at room temperature for at least 4 hours, or preferably overnight, to set completely.

9. Once set, invert the marshmallow onto a large cutting board dusted with the powdered sugar/cornstarch mixture. Cut into desired shapes using an oiled knife or cookie cutters.

10. Dust all cut sides with more powdered sugar/cornstarch mixture to prevent sticking. Store in an airtight container at room temperature.