Heat the oven to 160 C. Use foil or parchment paper.
Mix butter, cocoa, sugar, and heat everything. It's imperative to stir, so the mass doesn't burn constantly. Once everything melts and starts to boil, leave it to cool off.
Whisk vanilla essence with eggs and slowly pour flour. Slowly mix two masses into one, pour the nuts and put everything in the baking tin. Bake for about 20 to 25 minutes. Once there is no dough on the knife when you pull it out, it's ready.
Note. Can be served with fresh berries (Strawberries or blueberries).