Separate the egg whites from the yolks into 2 separate bowls.
Beat the egg whites with a pinch of salt and half of the sugar until a hard mousse is obtained.
Beat the yolks separately with the butter, the remaining sugar and the vanilla essence until everything is incorporated and a consistent composition is obtained. Add the milk and beat for a few seconds more.
Sift the flour and the baking powder and mix them in a bowl.
Gradually add the flour to the bowl with the egg yolks composition and beat at low speed until everything is incorporated.
Incorporate gently with a spatula the egg whites mousse in the obtained composition.
Prepare a narrow baking tin with baking paper and pour half of the batter.
Sift and mix the cocoa powder to the remaining batter, then use a spoon to gently add it to the middle of the baking tin (on top of the white batter).
Bake for 50 minutes at 180°C. Take out and check with a skewer to see if it's done - if the skewer comes out clean, then it's done, if there is batter on it, you need to bake it some more. I would say that no more than 10 minutes.
Take out of the oven and onto a cooling rack for a half an hour before serving.
Enjoy!