Place the sultanas, dates and rum in a bowl and leave to soak for 30 minutes.
Melt the chocolate, butter and golden syrup together in a pan. Remove from the heat, stir in the biscuits, orange rind and soaked fruit.
Grease an 18 cm (7 inch) square cake tin and line the base. Turn the mixture into the tin and chill for 1 hour.
Melt the plain dark and white chocolate for the topping in 2 separate bowls over pans of simmering water (see