Preheat oven to 180ºC. Grease or line a baking tray with baking paper.
Cream butter and sugar until light and fluffy. Sift flour and cocoa. Stir into creamed mixture. Fold in cornflakes.
Spoon mounds of mixture onto the tray, gently pressing together.
Bake for 15 minutes or until set. When cold ice with chocolate icing and decorate with a walnut if wished.