Crush Candy Canes: If not using pre-crushed candy canes, use a mortar and pestle or a zip-lock bag and rolling pin to crush them. Set aside.
Melt Butter: Over low heat in a saucepan, stirring occasionally. Alternatively, use a microwave in 30-second bursts. Transfer to a large bowl and cool until slightly warm.
Prepare Wet Mixture: To the cooled butter, add both sugars and whisk for 20 seconds. Then whisk in the egg and vanilla.
Incorporate Dry Ingredients: Sift in the flour, cocoa, cornstarch, and baking soda. Add the salt and combine until a thick dough forms.
Add Chocolate and Candy Canes: Fold in chocolate chips and crushed candy canes.
Chill: Refrigerate dough for 30 minutes. For overnight chilling, see notes.
Preheat Oven: To 356ºF (180ºC) fan-forced. Line a baking sheet with parchment paper.
Form Cookies: Scoop 45g/1.5 oz (or 3 level tablespoons) of dough, roll into tall balls, and place on the tray, spaced 10cm apart. Do not flatten balls.
Bake: 10-12 minutes until edges are set but centers are soft. Remove and tap the tray on your bench top or stove top 3 times to help slightly deflate.
Decorate: While still warm, add extra crushed candy cane to the top of each cookie.
Cool: Let the cookies sit on the sheet for 15 minutes, then move to a wire rack to cool completely.
Serve and Enjoy!