Chop up the chocolate into smaller pieces so that it will melt faster and mix it with the cocoa powder and salt. Warm the cream and water in a pan until simmering just under a boil.
Slowly pour a third at a time of the hot liquid over the chocolate and cocoa powder, whisking to emulsify everything together. Once all of the liquid is added you should have a pourable, smooth and slightly shiny ganache. If the mixture feels a bit thick or if it is not perfectly smooth you can warm it up very gently in the microwave or over a bain-marie to help get rid of any lumps and get it to the right consistency.
Pour the mixture into a cream siphon, attach the top and charge it with three gas charges. Shake the siphon very vigorously for 2 mins. Carefully spray the mixture out into a container. The ganache will now be very aerated. Once the siphon has been entirely emptied quickly put the mousse into the fridge to set for 2-3 hours.