Prepare a 9x13 pan by lining it with aluminum foil and spraying the foil liberally with nonstick cooking spray.
Place 1/4 cup plus 2 tbsp water in a small microwave-safe bowl, and microwave for 2 minutes. Pour 1/3 cup cocoa powder over the hot water, and stir or whisk to dissolve the cocoa.
Place 1/2 cup cold water in the bowl of a large stand mixer. Sprinkle the gelatin on top and stir briefly to distribute. Let gelatin sit and dissolve for at least 5 minutes.
Spoon warm cocoa mixture into gelatin, and stir or mix on low to thoroughly combine.
Place remaining 1/2 cup of water, corn syrup, and sugar in a medium saucepan over medium-high heat. Stir to dissolve the sugar, and insert a candy thermometer.
Allow the mixture to cook without stirring until it reaches 240 degrees on the thermometer. Brush down the sides occasionally with a wet pastry brush to avoid crystallization. While the candy cooks, briefly run the mixer on low once or twice to ensure the gelatin and cocoa is well-mixed.
Once the candy reaches the proper temperature, remove it from the heat immediately. Turn the mixer to low, and slowly pour the hot syrup into the mixer bowl. Be careful, as the syrup is extremely hot. If you have a large (at least 3 cup) liquid measuring cup with a spout, you can transfer the hot syrup to the cup before pouring to make it easier.
Gradually increase the speed of the mixer until it running on high. Whip the marshmallow mixture for 15-20 minutes, or until it is stiff and shiny. You can tell it is done when you stop the mixer and lift up the beater, the marshmallow will slowly drip back down into the bowl in a thick, shiny stream.
Pour the marshmallow into the prepared pan and smooth the top. Allow it to sit and firm up at room temperature for at least 10 hours.
Sift the powdered sugar, 4 tbsp cocoa powder, and cornstarch together. Cover your workstation with waxed paper to protect it, and liberally sprinkle the surface with the sugar/starch mixture. Sprinkle the top of the marshmallow with the sugar/starch coating, and flip the marshmallow face down onto the prepared surface.
Carefully peel back the foil from the marshmallow. Spray a large smooth-bladed knife with nonstick cooking spray, and coat both sides with the coating powder. Cut the marshmallow into squares, coating the knife blade with sugar/starch as needed. Roll the cut edges of the marshmallows in the coating mixture so that all sides are smooth and not sticky.
If desired, melt 12 ounces chocolate coating (or temper 12 ounces chocolate) and dip the cut marshmallows halfway into the chocolate. The chocolate needs to be able to set without refrigeration (which will ruin the texture of the marshmallows) so use only tempered chocolate or chocolate coating for dipping.
To save marshmallows, store them in a dry airtight container in a temperate, dry location. Do not refrigerate them or keep them in a very humid place. Fresh marshmallows go stale after about a week, so these are best eaten soon after they are made.