Melt butter in a pan and pour it into a large bowl.
Add cocoa powder, brown sugar, and granulated sugar to the bowl; whisk until mixed well.
Add egg, egg yolks, and peppermint extract; whisk until combined.
In a separate bowl, mix flour, salt, baking powder, and baking soda; then add to the wet mixture and stir until a dough forms.
Fold in chocolate chips and crushed candy canes.
Roll 100g of dough into balls and place them on a tray; repeat until all dough is used.
Chill the dough balls in the fridge for 1 hour.
Preheat the oven to 180°C (350°F) about 10 minutes before baking, then bake the cookies for 20 minutes, slamming the tray halfway through. Let them cool for 10 minutes before enjoying!