Preheat the oven to 400°F (200°C) and line an 8in (20cm) square brownie/baking tin with parchment paper.
Melt the dark chocolate in a microwave in 20-second intervals, stirring regularly to make sure it doesn’t burn.
In a large mixing bowl, with an electric handheld whisk, or in the bowl of a stand mixer fitted with the beater attachment, whisk together the melted butter and both sugars for 3 to 5 minutes until totally combined. Add the eggs, one at a time, mixing after each addition, then add the vanilla extract and mix again. Gradually add the melted chocolate and mix to combine. Add the flour and cocoa powder and fold them gently into the wet mixture with a rubber spatula or wooden spoon until just combined. Fold in the chopped chocolate.
Transfer the batter to the lined tin and bake in the oven for 25 to 30 minutes until the mixture has a shiny crust on top, and just a little hint of wobble in the middle.
Remove from the oven and leave the brownies in the tin on a wire rack until totally cooled, then remove and slice!
The brownies will keep well for up to 5 days in an airtight container, and can also be frozen.