Preheat oven to 180 °C/gas mark 4.
Grease a swiss roll tin (approximately 16cm x 26cm).
Cream margarine or butter, sugar and cocoa together.
Add flour and coconut and mix to combine (it makes quite a stiff mixture).
Put mixture into tin and press flat.
Bake for 25-30 minutes on a middle shelf.
Melt chocolate and spread over shortbread.
Cut into 12 pieces while still warm.
Leave to cool before removing from tin.