1. Preheat oven to 180°C. Lightly grease two baking trays or line with baking paper.
2. Cream butter and sugar together until light and creamy. Sift flour and cocoa together, add to the creamed mixture and stir until combined. Stir through the cornflakes.
3. Roll dessertspoonfuls of the mixture into balls and place on a baking tray, allowing room for them to spread. Press each ball lightly with a fork.
4. Bake for 12-15 minutes or until cooked. Remove from the oven and place on a wire rack to cool.
5. Make the chocolate icing by beating the butter, icing sugar and cocoa together. Add enough boiling water so the icing is just spreadable. Spread over cooled biscuits and decorate each with a walnut half.