In a bowl add the water and the gelatin together, mix and leave it to sit.
In a saucepan heat 100ml of the cream, the instant coffee and the sugar until just simmering. Turn off the heat and stir in the gelatin until dissolved.
Add in the chocolate and let it sit for two minutes before mixing together into a smooth chocolatey liquid. Leave it to cool for five minutes.
Whip the remaining 400ml of cream until soft peaks form, then add this in three lots, to the chocolate mix. Fold it in carefully and don't over mix.
Spoon on top of the cold brownie base, smooth it down, cover with foil and chill for at least 4 hours (or overnight.)