1. In the bowl of a stand mixer, using the whisk attachment, mix together the cold water and gelatin.
2. In a deep pot with a candy thermometer attached to the side, combine the honey, water, vanilla, salt. Heat the mixture over medium-high heat until it reaches 240 degrees F., the hardball stage (roughly 9 to 14 minutes).
3. Carefully pour this mixture into the gelatin mixture (it will be very hot).
4. Starting with the mixer on low speed and gradually increasing to high speed, whip for 8 to 10 minutes, until the mixture becomes white and thick. In the last 4 minutes, add the cocoa powder.
5. Meanwhile, line an 8-inch baking pan with parchment paper and lightly dust with cocoa powder.
6. As soon as the mixture has fully thickened and the chocolate is incorporated, quickly pour the marshmallow fluff into the pan.
7. Lightly dust with more cocoa powder (this will help you smooth out the surface).
8. Let cool for several hours in the fridge, which will allow the marshmallows to set.
9. Cut the marshmallows into squares. If you find that the knife is sticking, coat it in cocoa powder before each cut. Store the marshmallows in a sealed bag or container at room temperature, or freeze for later use.