We can use gelatine powder or gelatine sheets, it makes no difference. Soak gelatine (aka bloom) in water until it gets a rubbery consistency.
Chocolate
Preferably in a double boiler, heat water and dissolve the cocoa powder by slowly stirring it until smooth and lump free. A simple pot on the heater can also be used but mind not to burn the cocoa.
Once we get a smooth cocoa paste, slowly stir in the cream. Be careful not to create bubbles while stirring. This recipe will result a light colored mirror glaze. For darker glaze just replace parts of the cream or the whole of it with water.
Dissolve the rubbery gelatine and add any additional sweetener if required.
Filter the mirror glaze through a fine mashed sieve as a precaution. As we haven't used chocolate but only cocoa powder it shouldn't be any hard pieces floating around but it's better to stay safe then sorry.
Set it aside and let it cool to 86°F / 30°C. This part is important as too cold glaze will not spread as easily creating bumps and uneven surface while too hot glaze will run off the cake, not leaving thick enough layer to cover the cake.
Before pouring the glaze on a cake, check for any skin on top of the glaze. We used cream not milk so sometimes there isn't skin on top but if there is any, just strain the glaze again and then pour it over the cake. Also check for air bubbles and pour the glaze form a low height to prevent the creation of undesired bubbles in the glaze.