Preheat oven to 180C
Grease a 9 inch pan with a removable base with butter and cocoa powder and set aside
Melt chocolate and butter together either in microwave or bain marie (over a bowl of hot water)
While chocolate is melting separate your eggs (yolk together, white together)
Add the vanilla to the yolks and whisk
Once the butter and chocolate have melted and are homogeneously mixed remove a bit of the chocolate mix, add it to the egg yolks (to adjust the temperature so your yolks don't cook) and mix well. Once done, add the egg yolk mixture into the chocolate butter mixture and mix them thru
Add the sugar to the egg yolk chocolate mixture and mix them well
In a separate bowl, sieve the flour, coacoa powder, and baking powder. Mix them together then gradually mix them into the chocolate mixture
Beat the egg whites with a hand held machine or stand mixer until white and fluffy
Fold it into the chocolate mixture, do not over mix. Just until the ingredients are incorporated
Pour into greased pan and bake for 25 mins. Not one minute more
Remove from oven and let it cool down and come to room temperature before carefully removing it from the pan
Just before serving, cover with sieved powdered sugar