1. Combine cocoa, powdered sugar, and cornstarch in a small bowl.
2. Coat a 9-inch square glass or ceramic baking dish with cooking spray; dust pan with 2 1/2 tablespoons cocoa mixture. Reserve remaining cocoa mixture.
3. Combine 1/2 cup cold water and gelatin in a large bowl; beat with a mixer at medium speed until blended. Let stand 30 minutes.
4. Combine granulated sugar, corn syrup, salt, and 1/2 cup cold water in a small heavy saucepan. Cook over medium heat 5 minutes or until sugar dissolves, stirring occasionally. Increase heat to medium-high; cook without stirring until candy thermometer reaches 244°. Remove from heat.
5. Slowly pour hot sugar syrup into gelatin, beating at low speed until combined. Beat at high speed 12 to 15 minutes or until mixture triples in volume. Add vanilla; beat 30 seconds. Pour mixture into prepared pan; spread mixture in pan using a rubber spatula coated with cooking spray. Sprinkle marshmallows with chips, gently pressing chips with spatula to adhere. Top with 1 1/2 tablespoons reserved cocoa mixture. Let stand, uncovered, at room temperature overnight.
6. Run a knife around outside edge of pan; remove from pan. Cut into 1-inch squares.