Mix the butter and sugars. Add cold cubed butter, light brown sugar and caster sugar to your stand mixer. Beat until sandy and combined.
Add the egg and extracts. Mix in the egg, vanilla and peppermint extract.
Add the dry ingredients. In a separate bowl whisk flour, cocoa powder, cornflour, bicarbonate of soda and salt. Add to the mixer and blend into a thick dough.
Add the chocolate chips. Fold through until evenly distributed.
Chill the dough. Scoop into balls and chill for 30 minutes.
Bake. Heat the oven to 175 C and bake the cookies for 10 to 12 minutes. The edges should be set while the middles stay soft.
Cool. Leave on the tray for 10 minutes then move to a wire rack.
Decorate. Dip half of each cookie into melted white chocolate, then sprinkle crushed candy canes over the top. Leave to set.