Preheat oven at 350º.
In the bowl of your Kitchen Aid equipped with the whisk attachment, whip the butter until creamy. Add the sugar slowly and incorporate well. Add egg yolks one at the time.
With the motor running at slow speed, add the flour and then the milk. Add baking powder.
Beat the egg whites to soft peaks and incorporate them slowly to the flour mixture. Divide the creamy mixture into two bowls and add dark powder chocolate to one of the bowls. Mix well.
Butter a rectangle baking dish. Sprinkle flour to the baking dish to cover all the butter with a shadow by shaking the pan. Discard any excess flour.
Alternate dropping one tablespoon of cream mix, and one tablespoon of chocolate mix into the baking dish.
Bake in the oven for 30 minutes before checking with a toothpick inserted in the middle to check if the cake is done. The toothpick needs to come out easily and with some cake crumble attached to it, but not liquid. Do not overcook as it will dry out.
Remove from the oven and let it cool off.