Place 38 ml water and gelatin together in a small bowl to bloom gelatin. Let stand for 5 minutes.
Using a whisk, mix together cocoa powder and 2/3 cup of water in a medium saucepan on medium heat. Make sure all of the clumps are removed, and it should become the consistency of paste.
Add a couple of tablespoons of the heavy cream to mix into the cocoa powder. Whisk until it has combined.
Add the rest of the heavy cream and sugar and mix with a wooden spoon as gently as possibly. You will not need your whisk anymore, as this will create bubbles, which can ruin the chocolate mirror glaze.
Bring the mixture until it starts simmering. Remove the mixture from the heat source right before it starts boiling.
Add the gelatin and mix together.
Place a fine mesh sieve over a small heat proof bowl. You'll want the sieve as close to the bottom of the bowl as possible, this will prevent bubbles from forming.
Pour the mirror glaze into the sieve. Let the mixture fall through on its own. It will take awhile to fall through (20-30 minutes).
As soon as the glaze is through the sieve, put a layer of plastic wrap touching the glaze to cover it. This will prevent the glaze from forming a skin on it.
Cool the chocolate mirror glaze until it reaches room temperature. You can place it in the fridge until you're ready to use it.