Preheat oven to 180 degrees celsius. Line two baking trays with baking paper and set aside.
Cream butter and sugar on Speed 4, 30 seconds. Scrape and repeat 2 more times or until pale and creamy.
Add the plain flour and cocoa powder and mix on Interval (kneading function) for 20 seconds. Scrape down the sides and repeat.
Add the cornflakes and mix on Reverse, Speed 3, 5 seconds (use the spatula to help mix).
Chill the dough in the fridge for 30 minutes.
Shape tablespoonfuls of the mixture together and place onto the baking trays. Cook for 10-15 minutes.
Remove from the oven and allow to cool for 10 minutes on the trays before transferring to a wire rack to cool completely.
To make the chocolate icing, place all of the icing ingredients into the Thermomix bowl (using 2 tablespoons hot water). Mix on Speed 5, 15 seconds.
Scrape down the sides of the bowl and repeat for a further 10 seconds or until smooth.
Add an extra tablespoon of hot water if the icing is too thick to spread.
Place a small spoonful of chocolate icing onto the top of each biscuit and top with a walnut.