Preheat oven to 180°C bake. Line two large baking trays with baking paper.
In a large bowl, beat butter, Chelsea Caster Sugar or Chelsea Organic Raw Sugar and vanilla until light and fluffy.
Sift in the flour and cocoa. Stir until mostly combined, then add the cornflakes and mix well.
Roll tablespoons of mixture into balls and place on prepared trays. Press down lightly with a fork.
Bake for 15-20 minutes, until biscuits are just firm to the touch. Leave on trays for a few minutes, then transfer to a wire rack to cool completely.
Sift Chelsea Icing Sugar and cocoa powder into a bowl. Add butter and 1 tablespoon boiling water at a time, mixing well and adding more hot water as required until icing is a spreadable consistency. Ice cooled biscuits and decorate with walnut halves, if desired.