2 hours before, in a small bowl, combine the boiling water and 3½ tablespoons cocoa powder. Stir and mix well. Place in the fridge until cold.
In a medium bowl, whisk together ½ cup cocoa powder and ½ cup powdered sugar. Try to get rid of most of the big lumps.
Prepare a 9 x 13-inch baking pan, lightly grease with nonstick cooking spray. Use a paper towel to remove extra oil. Dust the pan with the cocoa and powdered sugar mixture very generously to cover. Set aside.
In a large mixing bowl with an electric mixer fitted with a whisk attachment, pour in the cold chocolate liquid and ¼ cup cold water. Sprinkle the gelatin over. Mix everything slightly with a fork. Set aside and let the gelatin bloom.
In a medium saucepan, combine the sugar, corn syrup, ½ cup water, and salt. Cook over medium-high heat with lid on for 3 minutes. Remove the lid. Attach a candy thermometer onto the side of the pan. Turn the heat to medium and continue to cook until the sugar mixture has reached 240˚F, about 7 – 8 minutes. Remove from heat immediately.
Mix the gelatin mixture quickly. With the mixer on low speed, slowly pour the sugar mixture down the side of the bowl into the gelatin mixture. Once all sugar mixture has been added, increase the mixer to high speed. Whip until the mixture turns very thick and is lukewarm, about 12 – 15 minutes. Add the vanilla extract at the last minute of whipping.
When the marshmallow mixture is ready, quickly transfer to the prepared pan with a spatula. Spread the mixture evenly. Dust with more cocoa and powdered sugar to cover. Save the rest of the cocoa and powdered sugar.
Let the marshmallows to rest at room temperature uncover for at least 6 hours, best for overnight.
Prepare a cutting board lightly cover with cocoa and powdered sugar. Run a thin knife around the edges between the marshmallow and the pan. Turn the marshmallows out onto the cutting board. Lightly grease a large sharp knife with nonstick cooking spray. Wipe off extra oil. Cover the knife with the cocoa and powdered sugar. Cut down firmly into the marshmallow and make ¾-inch squares or desired shapes. Cover all sides of the marshmallow squares with more cocoa and powdered sugar until not sticky.
For storage, keep the marshmallows in an airtight container for up to 1 week (maybe even longer).