Preheat the oven to 170°C (fan). Line a 20cm baking tin with parchment paper.
Place the dark chocolate and butter into a small heat-proof bowl and melt together over a pan of water. Leave aside to cool.
Put the eggs and brown sugar into the bowl of your free-standing mixer. Whisk until light in colour and doubled in volume. We used the paddle/K beater for this.
Add the melted butter and chocolate to the egg mixture and whisk again, just enough to combine.
Sift the flour and cocoa powder together into a large mixing bowl. Then add this to the wet mixture, followed by the chocolate chips.
Fold the dry ingredients into the wet either with a spatula or using your mixer on a low setting (this is how we do it).
Pour the batter into the tin, spread it out and bake in the preheated oven for 25-35 minutes. Ovens vary, so check after 25 minutes. When the sides of the brownies are set and there is a slight wobble in the centre, they are baked.
Allow the brownie slab to cool completely in the tin. We often add them to the fridge for an hour when they are cooled just to make cutting them even easier.
Once ready to cut, remove from the tin in one piece, score and cut into 12 servings.
Store your brownies in an airtight container for up to five days. If your kitchen is very hot, keep them in the fridge. These brownies also freeze very well.
