Mix flour and cocoa powder, sieve.
Melt dark cooking chocolate and butter. You can use microwave (like I did here) with medium heat for 1 min, stir and burst 1 more minute. Or you can melt in a small pot that placed in a bigger pot filled with boiling water. Stir until melted thoroughly.
Beat 4 eggs until frothy, add sugar. Beat again until those sugar felt dissolved. You will notice that the eggs will look pale
Add cocoa flour to beaten eggs, mix well
It will be a little more harder to whisk, so use a spatula
When it's all mixed, add the chocolate butter to the mixture. Mix well.
Grease your baking dish with little butter and dust with flour. remove excess flour.
Pour your brownies mixture into baking dish, and put it in preheated oven. Bake until done. You can stick a toothpick in and pull it out, if it comes clean (no dough on it) then it's done.