Melt the chocolate in a heat-proof bowl over a pan of simmering water or in the microwave in 10 second increments on high. Stir to melt down any remaining lumps, taking care not to over heat the chocolate. Set aside to cool.
Beat the butter and sugars together using a hand mixer on medium speed until light and creamy. Add the egg and vanilla and beat briefly to combine.
Now add the melted chocolate and and beat on low speed until smooth. Sift in the flour, baking soda, salt and cocoa. Fold gently with a silicone spatula or wooden spoon to combine into a thick, sticky dough, then stir in the chocolate chips.
Cover the bowl with clingfilm and chill for about 30 minutes to 1 hour, or until firm enough to scoop, but not super hard. If you find that the dough has hardened considerably, let it sit at room temperature for 15 to 20 minutes before you scoop it out. Otherwise, I find the cookies barely spread and don't have the perfect brownie-like softness in the center. Colder dough creates crinklier tops if you prefer that!
Preheat the oven to 170 C and line a baking tray with a silicone baking mat.
Use a medium cookie scoop (1.5 tablespoons) to scoop out 20 to 22 portions of the dough (see notes). Place them at least an inch apart on the baking tray (you might need more than one tray depending on your oven size, I bake one batch of 9 cookies at a time).
Bake for 10 to 12 minutes until the cookies have spread a bit, but are also still a little puffy. Very gently press down on the tops with the back of a spoon to flatten further, then let them cool on the tray and firm up for about 20 minutes. Transfer to a wire rack to cool completely. This creates soft, chewy centers even 4 to 5 days later. If however you prefer your cookies crisper and drier overall, bake for 15 minutes.
Store in an airtight tin at room temperature for a week. If it's very humid where you are, store them in the fridge after the first couple of days. Cookies freeze beautifully too! You can either bring them to room temperature, or additionally gently reheat them before eating. Happy baking!