Preheat oven to 180 degrees. Line a baking tray with baking paper,
Beat sugar & butter till creamy. Add vanilla. Mix in flour, cocoa & baking powder, then add cornflakes. Place heaped teaspoonfuls on tray & bake for 12-15 minutes. Allow to cool before icing.
For icing
Beat icing sugar, butter, water and cocoa till smooth & spread over biscuits. Top with a walnut half. Store in airtight container.