Add melted butter, vanilla seeds and sugar in a bowl and mix (c. 2 min with mixer.
Separate the egg yolk from the egg whites and keep both separate. Beat the egg whites with an mixer in a clean bowl until you can hold the bowl upside down without any egg whites falling out.
Add the egg yolks one by one to the butter/sugar mix while mixing.
Add the flour, pinch of cinnamon and pinch of a salt gradually to the mixture while stirring with a spatula.
Carefully and gradually fold the beaten egg whites into the batter.
Grease the cake pan with butter and dust with flour.
Peel the apples and cut them into thin slices.
Pour just enough batter into the cake pan to cover the bottom, then arrange a layer of apple slices on top. Cover the apple slices with batter.
Add another layer of apple slices and cover with the remaining batter. Finish by gently pushing the remaining apple slices partly into the batter.
Pre-heat the oven at 175 °C (350 °F). Add the cake for 15 min. After 15 min, reduce the temperature to 160 °C (320 °F) and bake for another 35-40 min. Check whether the cake is ready by pushing a knife into the middle of the cake. If you remove the knife and batter sticks to it, it is not yet ready (see image for an example of when it is not ready).
