Preheat your oven to 150°C fan bake. Line a 20cm x 20cm slice tin with baking paper.
Whisk the melted butter and cocoa until smooth in a stand mixer or using an electric beater.
Add the sugar and the eggs. Mix until combined.
Fold in the flour, baking powder and chocolate chips. Transfer to the prepared tin and spread out evenly.
Bake for 40 minutes, rotating the tray in the oven at the 20-minute mark. The brownie will be slightly undercooked at this stage, which is perfect. Remove from the oven and leave to cool before cutting.