Beat the butter and brown sugar together until light and fluffy.
Add the eggs one at a time, mixing well after each addition.
In a separate bowl, combine the flour, baking powder, and vanilla sugar. Add this dry mixture to the wet ingredients and stir to combine.
Add the milk and mix until the batter is evenly combined (the batter will be thick).
Pour the batter into the greased pan and spread it evenly toward the corners.
Arrange the prepared apple slices on top in a pattern you like. Sprinkle with pearl sugar, if using.
Bake for around 30–35 minutes, or until the cake is golden and a toothpick inserted into the centre comes out clean.
Remove from the oven and serve warm, or allow to cool completely. Enjoy!