Preheat the oven to 425 degrees F (220 degrees C). Grease a deep baking pan with butter or line with parchment paper.
Toss apple slices with lemon juice in a bowl and set aside.
Combine butter, sugar, and vanilla sugar in a large bowl and beat using an electric blender until pale and fluffy. Add eggs, one at a time, and mix well after each addition. Combine flour, baking powder, and salt together in a separate bowl and mix on low speed. Fold apple slices into the batter.
Spread batter into the prepared baking pan and smooth the top.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 30 minutes.
