Preheat the oven to 180°c. Line two sheet pans with baking paper.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar until light and fluffy. Add the vanilla and mix to incorporate. Add flour, cocoa, and salt, and mix until combined. Add the cornflakes and mix briefly to incorporate.
Place 2 Tbsp scoops on the prepared baking sheets, leaving room for spreading. Gently roll each piece of dough into a ball and flatten slightly.
Bake for 12-15 minutes, rotating pans half way through, or until the edges are set. Remove from oven, and allow to sit on the tray for 5-10 minutes. Move to an oven rack, and allow the biscuits to cool completely.
Combine butter, icing sugar and cocoa in a small mixing bowl. Add boiling water and mix until smooth (refrain from adding extra water until you have mixed well, or you get stuck in an icing sugar - water - icing sugar vortex)
Spoon a small amount of the icing onto each biscuit, and place a walnut half on each one.
Store in an airtight container in the fridge for a more 'fudgy' texture, or out at room temperature if you prefer them more crunchy!