Heat your oven to 350 (180C) and line a cookie tray with parchment paper.
Take a medium sized saucepan and add the butter, sugar and cocoa powder. Place on a low heat until the butter melts.
Mix in the flour and cornflakes.
Spoon the mixture into 12 even balls pressing together with your hands and pat them down on the cookie tray. Bake for around 18 minutes until set. Let sit on the tray for 5 minutes before transferring to a wire rack to cool.
To prepare the chocolate frosting take a small saucepan and melt the butter over a low heat, stir in the cocoa powder and icing sugar and mix together with a small whisk, then beat in enough of the hot water until the desired consistency is obtained. When the cookies are cold top with chocolate frosting and a walnut half.