Combine the wheat flour and baking powder. Stir together the oil, eggs, yoghurt, quark, salt, sugar, and vanilla sugar. Add the flour to the egg mixture and mix everything together to form a smooth batter. Crumble the biscuits and fold in. Peel and quarter the apples, brush with lemon juice. Score the outside of the apples lengthways a few times to produce a fan. Pour the battle into the tin. Press the apple quarters gently into the batter with the inside facing downwards.
Bake the apple cake at 180°C with top and bottom heat for around 50-55 minutes. Cover for the last quarter of an hour.
Toast the flakes in a frying pan with a little oil. Sprinkle with sugar and allow to caramelise. Scatter over the still warm apple cake.