Preheat the oven at 15 degrees Celsius.
Separate the egg whites and the egg yellows from each other. Place in 2 different bowls.
Whip the egg whites to a hard foam.
Whip the egg yellows, the sugar, water, vanilla sugar and the cacao powder together.
Melt the margarine and add in the egg yellow mixture while stirring.
Heat up the milk to lukewarm. And add gently to the mixture while stirring.
Add the flour while stirring.
Fold gently the egg whites foam in the mixture. No worries about the lumps. It´s normal.
Butter a baking form and pour the batter.
Place in the middle of the oven for 1 hour.
Take out the cake and let it cool down for 1 hour. Place in the fridge for 2 hours.
Cut in small pieces before serving.