Chocolate Marshmallows

For a more pronounced chocolate flavor, increase the amount of cocoa powder in marshmallows.
Pour 1/2 cup of water into a small microwave-safe bowl and sprinkle with gelatin.Combine remaining 1/2 cup water, sugar, corn syrup, and salt in a medium saucepan over medium-high heat; bring to a boil, stirring occasionally. Cook, without stirring, until the candy thermometer reads 250°. Pour the sugar mixture into the bowl of a stand mixer; let stand until the candy thermometer reads 210°C.Heat the gelatin mixture in the microwave on high power for 20 seconds or until the gelatin is melted, stirring after 10 seconds. Using a mixer on low speed, beat the sugar mixture with the whisk attachment; gradually pour the gelatin mixture into the sugar mixture in a thin stream. Add 1 teaspoon vanilla. Increase the speed to maximum; beat the mixture on high speed until light and fluffy (about 5 minutes). Reduce the speed of the mixer to medium and gradually add 1/4 cup cocoa; beat until smooth. Using a spatula greased with cooking spray, place the mixture on an 11-inch x 7-inch baking dish greased with cooking spray; smooth the surface. Let stand for 2 hours.Sift together the powdered sugar, cornstarch, and 2 teaspoons of cocoa into a jelly roll tin. Using a spatula smeared with cooking spray, remove the marshmallows from the mold; roll in the sugar mixture. Using scissors well greased with the powdered sugar mixture, cut the marshmallows into 78 (1-inch) squares. Sprinkle with powdered sugar; beat to remove any excess sugar mixture.Place the marshmallows on a cooling rack located on a baking sheet with sides. Place the bittersweet chocolate in a small bowl suitable for a microwave oven; reheat in the microwave on high power for 1 minute or until it melts, stirring every 20 seconds until smooth. Drizzle the marshmallows with melted chocolate; let stand until the chocolate is set.