Prepare an oven tray by greasing or covering in baking paper
Cream butter & sugar for 4-5 minutes until light and creamy
Add flour, cocoa & vanilla and mix well
Add cornflakes and lightly combine.
Take tablespoon size amounts and roll into balls spread evenly on the tray. Lightly flatten with your fingers.(I use a small cookie scoop, it is much faster and easier, see notes for further information)
Bake for 12 minutes
Allow to stand for 5 minutes before transferring to a wire rack to cool completely
In a medium bowl combine icing sugar, cocoa, vanilla & melted butter and mix
Slowly add the water a little at a time until the desired consistency is reached. If you accidentally add too much water just add a little icing sugar.
Ice the biscuits and finish with a walnut half. Allow the icing to firm up.