Preheat your oven to 160°C/320°F (or 140°C/285°F 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 5 minutes.
Add the sugar and eggs to a large bowl. Using an electric whisk, whisk for a few minutes. The mixture should look pale and silky and doubled in size.
Add the melted chocolate mixture into the bowl with the sugar and eggs then gently fold them together.
Add the flour, espresso powder and cocoa powder then mix gently until well combined.
Stir in the chocolate chips.
Transfer the mixture to your lined tin and spread it out evenly.
Bake for around 30-35 minutes until the brownies have risen and are pulling away from the tin slightly.
Let the brownie cool to room temperature then transfer to the fridge overnight to set.
Once set, remove from the tin, cut into squares and enjoy!