Preheat your oven to 160℃ (140℃ 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.
To a large bowl, add the sugar, eggs and vanilla extract. Using an electric whisk, whisk it all together for a few minutes- it should have increased in volume and look pale and silky.
Fold the melted butter/chocolate mixture into the sugar and egg white mixture.
Add the flour and the cocoa powder and mix gently until well combined. Then stir in all of the chocolate chips.
Bake for around 35-40 minutes until the brownies have risen and are pulling away from the tin slightly. There should be a slight wobble in the middle.
Leave the brownies in the tin until cool then transfer them to the fridge overnight to set. This will make them ultra fudgy and chewy.
Once set, remove them from the tin, cut into squares and enjoy!