Lightly grease a 20x20cm/8x8" baking tray with butter and line it with parchment paper.
Sift the cocoa, brown sugar, caster sugar and baking powder into a large bowl. Add the white and chocolate chips.
Melt the butter in a small saucepan and lightly beat the eggs.
Add eggs, butter and vanilla to the dry ingredients.
Stir until combined with a wooden spoon or spatula.
Pour brownie mix into your prepared tin and bake for 25-30 minutes, or until it's firm to the touch. The brownie should be crunchy on top. It will continue to cook as it cools and becomes firmer as well.
Cool completly then cut into 12 even squares and serve.
If you have a scale, use the scale and convert the units to oz if using imperial measurements. Otherwise use the cups version.