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 (200 g), at room temperature and sugar (100 g) until light and fluffy. Add the vanilla (½ tsp) and mix to incorporate. Add flour (180 g), cocoa (30 g), and salt (½ tsp), and mix until combined. Add the cornflakes (70 g) 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 (30 g), at room temperature, icing sugar (125 g) and cocoa (15 g) in a small mixing bowl. Add boiling water (1 Tbsp) 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 pecan 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!