Preheat the oven to 150°C.
Sift cocoa into a large mixing bowl. Add melted butter, sugar, eggs and vanilla. Mix to a smooth paste.
Sift in flour and baking powder, then add chocolate chips.
Pour mixture into a baking paper lined 20x30cm Swiss roll tin and bake for 65 minutes. Serve with lashings of Puhoi Valley custard.
* If you want a slightly taller chocolate brownie you can use a 20x20cm pan at the same temperature, but it will take at least 80 minutes to cook.