Pre-heat the oven to 170° Celsius (325° F).
Break the chocolate to pieces and set to melt together with the butter in a bain marie system.
Sieve the cacao, flour and the baking powder into a bowl.
In another bowl mix the brown sugar with the eggs with a mixer, until well blended.
Add the sieved dry ingredients and the vanilla extract and mix again.
Finally pour in the melted chocolate and butter mixture and mix all together.
Use a 20 centimeter square baking dish and spread butter in the bottom and the sides, so that the brownie will not stick to the dish.
Pour in the brownie batter and make sure to make the batter flat by tapping the baking dish gently to your work top.
Bake for about 40 minutes.
Take out of the oven when time is up and let rest for 5 minutes and serve.