Soak the gelatin in a bowl of cold water for 10 minutes before starting.
If using powdered gelatin, rehydrate it with the indicated amount of water. Stir and place in the refrigerator for about 20 minutes to let it bloom.
Pour the water and sugar into a saucepan and bring to a boil until it reaches 103°C (217°F). Remove from the heat as soon as it reaches the temperature.
Pour the syrup over the previously sifted cocoa powder. Mix well with a whisk, being careful not to whisk vigorously to avoid creating air bubbles.
Bring the cream to a boil, then remove from the heat and add the squeezed gelatin. Stir well until fully dissolved, then pour it over the syrup and cocoa mixture.
Blend with an immersion blender in a tall, narrow container. Strain through a fine sieve. Cover with plastic wrap directly touching the surface and let rest until the next day.
You can also use it the same day without issue, but letting it rest helps eliminate stubborn air bubbles.
The next day, gently reheat the glaze in a bain-marie without exceeding 40°C (104°F). Blend again and strain.
Let the glaze cool down to around 30°C (86°F), more or less depending on its viscosity, then glaze your frozen entremets straight out of the freezer.
Note: Always check the glaze’s fluidity before pouring it over the mousse cake.
If it's too cold, it will be thick, won't flow properly, and will result in a thick layer. If it's too hot, it will run off without properly covering the cake.
I recommend doing a test on the back of a frozen ladle. The glaze should spread evenly and form a thin, smooth layer over the surface.