Lightly grease the bottom and sides of a 20cm loose-bottomed cake tin. Mix together the melted butter and crushed biscuits and press into the bottom and sides of the greased cake tin. Place the tin into the freezer to chill while you prepare the chocolate filling. To make the filling, simply melt down the dark chocolate and mix in the contents of the pre-cooked tin of caramel. Quickly remove the cake tin from the freezer, and pour over the chocolate caramel filling. Spread out evenly using a spatula and place the tin in the fridge for an hour, to cool the chocolate and set the filling. When the filling has set, remove the tin from the fridge and put aside. Whip the cream to stiff peaks in a large bowl with the icing sugar and then fold in ½ of the bananas. Pour this mixture over the set chocolate caramel and spread out. At this stage, you may chill the banoffee pie for another 30 minutes. Finally when you are ready to serve, carefully remove the banoffee pie from the cake tin and place on your serving plate. Arrange the remaining banana slices over the top and dust with cocoa powder.