Break chocolate up and melt in a double boiler until completely melted. Add 300mls of cream and mix well. Whip the remaining 300mls of cream. On top of the 1st piece of sponge put some of the boysenberries- with some of the juice. Cover with whipped cream and then some of the chocolate/cream mixture. Repeat with 2nd and 3rd layers- using all of the boysenberries. Cover 4th layer with the last of the whipped cream, and finally with the last of the chocolate/cream mix, covering the sides aswell. Freeze for 1½ hours or refridgerate for at least 3 hours before serving.
