Preheat the oven to 200°C (350°F) and line two large baking trays with parchment paper.
To a large mixing bowl, add the melted butter and both sugars. Stir together using a wooden spoon.
Add the egg and vanilla to the creamed mixture and mix again until well combined.
In a bowl, sift together the baking soda, salt, and flour. Add the flour mixture to the cookie dough batter and mix with a wooden spoon until a homogeneous dough forms.
Add the roughly chopped dark chocolate and whole Smarties to the dough. Mix through. Chill the cookie batter in the fridge for 20 minutes.
Portion the dough into eight large cookies using an ice cream scoop. Place the cookies onto parchment-lined baking sheets, giving generous spacing between the cookie dough balls.
Bake one tray of cookies at a time in the center of theoven for about 8 minutes, or until the cookies are a light golden brown, their edges are crisp, and they have spread slightly. Remove the cookies from the oven, sprinkle them with a bit of sea salt, and press in some extra Smarties while the cookies are still warm. Allow the baked cookies to cool on a wire rack. Serve and enjoy!
Notes
Test the temperature of your oven before baking, if it is too hot, the cookies will burn on the bottom.
Each cookie should weigh around 70g.
Use a small bowl or chef's ring to perfect the shape of your cookies after baking.