Mix the gelatin and cold water in a small bowl, let is gel for about 1 minute. Get the boiled water, just microwave water in a cup until it starts to bubble, then measure out 2 tablespoons of it and mix it in the gelatin mixture.
While the gelatin mixture is cooling a little, get a mixing bowl and combine the sugar, cocoa powder, whipping cream, and the vanilla. Beat with an electric mixer until soft peaks form. Slowly pour in the liquid gelatin mixture while beating with the electric mixture until combined.
Pour the mousse into your desired serving containers and refrigerate for about half an hour. This recipe makes about 2 cups of mousse when finished.