cut the mars bars into small pieces. melt the butter and mars bars in a fairly large pot
once they are totally melted and start to look separated fold in the rice crispies until they are totally coated.
spread out into a shallow baking tray (about 1-2 cm deep)
leave to cool/put in fridge
melt cooking chocolate in bainmarie and spread over the top. leave to cool/put in fridge
cut into pieces and serve.