1. Pour 1/2 cup water into a small microwave-safe bowl, and sprinkle with gelatin.
2. Combine remaining 1/2 cup water, sugar, corn syrup, and salt in a medium heavy saucepan over medium-high heat; bring to a boil, stirring occasionally. Cook, without stirring, until a candy thermometer registers 250°. Pour sugar mixture into the bowl of a stand mixer; let stand until a candy thermometer registers 210°.
3. Microwave gelatin mixture at HIGH for 20 seconds or until gelatin melts, stirring after 10 seconds. With mixer on low speed, beat sugar mixture using a whip attachment; gradually pour gelatin mixture in a thin stream into sugar mixture. Add 1 teaspoon vanilla. Increase speed to high; whip mixture at high speed until light and fluffy (about 5 minutes). Reduce mixer to medium speed, and gradually add 1/4 cup cocoa; beat until combined. Using a spatula coated with cooking spray, scrape mixture into an 11 x 7-inch baking pan coated with cooking spray; smooth top. Let stand 2 hours.
4. Sift together powdered sugar, cornstarch, and 2 teaspoons cocoa into a jelly-roll pan. Using an offset spatula coated with cooking spray, remove marshmallow from pan; place in sugar mixture. Using scissors well coated with powdered sugar mixture, cut marshmallows into 78 (1-inch) squares. Dust with powdered sugar mixture; shake to remove excess sugar mixture.
5. Arrange marshmallows on a cooling rack placed on a rimmed baking sheet. Place bittersweet chocolate in a small microwave-safe bowl; microwave at HIGH for 1 minute or until melted, stirring every 20 seconds until smooth. Drizzle melted chocolate over marshmallows; let stand until chocolate is set.