Well grease a 15cm(6") cake tin. Put butter or margarine, syrup, cocoa and
raisins into a pan (or bowl) and heat gently until butter has melted. Stir
in crushed biscuits. Press into tin and leave in a cool place to set.
Melt chocolate and spread over cake. Once chocolate has set, remove from
tin and cut.

I usually used double quantity and put in a tray bake tin.

(Make double to allow for crumbage to be eaten
before it hits the plate! - TraceyX)