Unpack the cake layers and place one on the platter you will serve the cake on. Whip the cream until it forms stiff peaks. Dust the cocoa powder through a sieve into the whipped cream and beat it in. This ensures there are no cocoa lumps in the cream. Crush the bus cookies and fold them into the whipped cream. Spread a layer of bus cookie cream on top of the cake layer on the platter. Place the next layer over and spread another layer of the cream on. Finish with the last layer and spread the remaining cream on top and down the sides of the cake. Chop the chocolate and decorate the cake with it. Refrigerate the cake for 1-2 hours before serving.