In a mixing bowl, add butter, sugar, vanilla sugar, and eggs.
Combine the ingredients using an electric mixer or manually with a whisk.
In a separate bowl, add baking powder to the all-purpose flour and mix them.
Thereafter, pour the dry ingredients (lemon zest, baking powder, and flour) into the egg mixture. Mix everything.
Grease a springform cake pan with some butter and spread some flour all over the pan. This is to prevent the cake from sticking to the pan. Preheat the oven to 180 °C.
Peel and cut the apples into quarters. Remove the seeds from the apples. After which, slice the apple into roughly 0.5 cm thick slices. It is optional to distribute some lemon juice over the apple slices, in order to prevent browning.
Transfer the cake mixture into the springform cake pan and layer apple slices on top of the batter.
Depending on the oven, bake the apple cake for roughly 50 minutes.
After baking, take out the Apfelkuchen from the oven. Use the toothpick trick to check, if your cake is ready. Let it cool down for about 20 minutes before trying to get the apple cake out of the cake tin.
Before serving, spread some icing sugar and cinnamon powder (optional) over the German apple cake.
The Apfelkuchen can be enjoyed warm or at room temperature.