Preheat your oven to 160°C (140°C for fan ovens) and line an 8x8-inch tin with baking paper.
Melt the dark chocolate and the butter together over low heat on the stove then set aside to cool for around 10 minutes.
Add the brown sugar and eggs to a bowl, then use an electric whisk to mix them together for a few minutes until pale and silky.
Pour the melted chocolate mixture into the bowl then gently fold it into the sugary egg mixture.
Stir in the flour and cocoa powder to make a glossy batter, then add the chocolate chips.
Transfer the mixture to your lined tin and spread it out evenly.
Bake for around 25-30 minutes until the brownies have risen and are pulling away from the tin slightly.
Let your brownie cool to room temperature then transfer it to the fridge overnight to set.
Once set, remove from the tin, cut into squares and enjoy!