Grease a 13×9” or 9×9” square baking pan with canola oil (or any flavorless oil). Whisk together the water, salt, and gelatin in the bowl of a stand mixer and set aside to firm up.
In another bowl, whisk together the boiling water, cocoa and vanilla until it’s smooth and creamy, like a ganache. Set aside.
Weigh the sugar and corn syrup in a deep pot and add the water and stir. Clip a candy thermometer to the pot and set it over medium heat. Cook until the temperature reaches 250 F (it really should get to 250, not 245!).
Begin whisking the gelatin mixture on low and slowly pour in the hot sugar with the mixer on. Turn the speed to medium high and add the chocolate mixture. Let whip for about 10 minutes, until the marshmallow fluff gets stringy and begins to pull away from the bowl. Transfer to the prepared cake pan and smooth into one layer. Cover with foil and leave overnight at room temperature or set in the fridge for 3 hours to firm up.
Whisk together the powdered sugar and cocoa into a large bowl and dust the countertop with some of it. Transfer the marshmallow to the counter and using an oiled chef’s knife, slice into cubes. Toss the cubes in the bowl so you can coat all the edges.
Store marshmallows at room temperature in an airtight container. They’ll last about 3 weeks.