Preheat the oven at 150 degrees Celsius.
Divide the egg yellows from the egg whites in two different bowls.
Mix with an electrical mixer the egg yellows together with the sugar, vanilla sugar and the water. Mix until the batter becomes white.
Melt the butter and add it in the batter.
Add the milk slowly in the batter while stirring with the electrical mixer.
Mix the egg whites to a hard foam.
Add the liquid batter slowly while stirring with a big spoon in the egg white foam. It should still be small pieces of the egg white foam in the batter.
Place a baking paper in a baking form so it cover the bottom end the edges.
Pour the batter in the baking form, and place it in the oven for fifty minutes.
Take out the cake from the oven and let it cool down in room temperature.
Place the cake in the fridge for two hours before serving.
Before serving the cake make sure the soft side is placed downwards. You need to flip the cake upside down.