1. Add the butter and sugars to a large mixing bowl and mix on high speed for about 5 minutes until light and fluffy. Add the egg and vanilla extract and mix until well combined. Mix the flour with cocoa powder, cornstarch, baking soda, and salt in a separate bowl, then add to the mixing bowl and mix until just combined. Add the chopped chocolate (keep some chocolate to decorate) and fold in. Place the dough in the fridge for about 30 minutes.

 

2. Preheat the oven to 410°F (210°C). Line a baking sheet with baking parchment. Divide the dough into four portions, shape into balls and place with some space in between on the baking sheet. Press the leftover chocolate into the dough balls and bake for 12-13 minutes until the top of the cookies is dry and looks almost set. Take out of the oven and let cool down for about 20 minutes on the baking sheet, then transfer to a wire rack and let cool down completely.