Butter an 18cm (7in) sandwich tin. Put the biscuits in a plastic bag and with a rolling pin, smash into little uneven crumbs.
Put the butter and syrup in a bowl and melt in a microwave for 1 ½ minutes. Stir in the cocoa and sultanas, then slowly stir in the biscuits and mix thoroughly. Spoon into the tin and press down firmly.
Melt the chocolate over a pan of simmering water or micro-wave on medioum for 2 minutes. Pour over the biscuit mixture and chill for 30 minutes. Slice and serve.