Preheat your oven to 180°C. Grease or line a baking tray with baking paper.
In a large bowl beat butter, sugar and vanilla until light and fluffy. Sift in flour and cocoa, stir thoroughly with the butter mixture until combined before stirring in the cornflakes.
Place heaped teaspoonfuls onto a baking tray, squeeze mixture together gently if necessary then press lightly with a fork.
Bake for 15-20 minutes. Cool on a wire rack before icing.
Icing: Mix chocolate icing sugar, butter and 1 Tbsp boiling water at a time until combined, adding more hot water if necessary until icing is a spreadable consistency. Ice biscuits and top each one with half a walnut.