Preheat Oven: Preheat the oven to 180ºC (356ºF). Line a baking tray with parchment paper and set it aside.
Combine Dry Ingredients: In a mixing bowl, combine flour, cocoa, baking powder, and baking soda. Mix well and set aside.
Mix Wet Ingredients: Using a hand mixer or a stand mixer fitted with the whisk attachment, whisk together butter and sugar for 1 minute.Scrape down the sides.
Add Egg and Vanilla: Add the egg and vanilla, beat for 20 seconds.
Incorporate Dry Ingredients: Add the flour mixture and beat until it begins to form a soft dough, about 30 seconds. It will turn crumbly but continue beating until the dough forms.
Form Dough: Using your hands, form the mixture into a smooth dough. Wrap the dough in cling film and refrigerate for 15 minutes.
Shape Cookies: Pick up 1 tablespoon (30g) of dough and roll it into a ball. Place it on the prepared tray and press down slightly, ensuring a 1-inch gap between each cookie. Repeat with the remaining dough.
Add Smarties: Press 5-8 Smarties onto the top of each cookie.
Bake: Bake for 10-11 minutes until cracks have formed on the top and the centres are set.
Cool on Tray: Allow the cookies to cool on the tray for 2 minutes.
Transfer to Wire Rack: Move the cookies to a wire rack to cool completely.