In a small bowl, combine the gelatin and water. Set aside to bloom for at least 10 minutes.
Bring a pot of water to a low boil. Add the chocolate to a heat proof bowl that fits over your pot. Place the bowl of chocolate over the pot and stir until melted. Remove bowl from heat.
In a small saucepan, bring 1/4 cup of the cream for a boil. Stir the gelatin mixture into the hot cream until fully dissolved. Remove from heat.
Whisk the cream/gelatin mixture to the melted chocolate pouring in about 1/2 of the mixture at a time. Whisk until smooth.
In another mixing bowl, use an electric mixer to beat the remaining 1 cup of heavy cream until soft peaks form. Once the chocolate mixture has cooled, carefully fold it in with a rubber spatula.
Pour the mousse over the brownie layer and use the rubber spatula to smooth out the top. Refrigerate for at least 6 hours.
To serve, lift the mousse brownies from the pan using the parchment paper edges, sprinkle with a little cocoa powder (optional), run a knife through cold water and cut into square bars. You may need to wipe down the knife and run it under cold water again between slices for a clean slice look.