Melt the butter in a microwave safe mug for around 10-20 seconds.
Mix in the sugars, vanilla and peppermint extract with the melted butter. Stir in the egg yolk until fully combined.
Add in the flour and cocoa powder, and mix until fully incorporated.
Place in the microwave, and microwave for around 45 seconds. Allow to cool.
Frost with icing, and top with the crushed candy cane.