Line the base of an 18cm (7in) loose-bottomed deep flan tin or round cake tin with non-stick baking parchment. Break the chocolate into small pieces. Place the plain or milk in one bowl and the white in another. Add 2 ounces of butter to each. Stand bowls over pans of hot water. Leave until chocolate and butter have melted, stirring occasionally. Place bowls on work surface and stir half the cream, nuts, dates and coarsely crushed biscuits into each. Spoon the darker mixture into the tin and spread level with the back of a spoon, pushing the mixture down into the corners. Top wih the white mixture. Cover with foil or cling film and chill until set. Remove from tin and serve cut into slices.