Pre-heat oven to 180 degrees (160 degrees celsius fan-forced). Line a 4cm deep, 20 x 20cm baking tray.
Using an electric mixer, beat the butter and sugar until pale and creamy. Add eggs one at a time until well combined.
Stir in the flour, cocoa and baking powder, mixing until well combined. Pour into prepared tray and sprinkle with chopped chocolate. Bake for 25-30 minutes until just cooked. Allow to cool slightly before removing from tray.