Preheat oven to 200 C / Gas 6. Line a rimmed baking tray with greaseproof paper.
For the pastry combine sugar, butter, and vanilla sugar in a large bowl. Beat with an electric mixer until smooth and creamy. Add eggs one at a time, beating well after each addition. Beat in lemon zest with the last egg.
Combine flour and baking powder and sift over butter mixture. Quickly beat into mixture with the electric mixer running on low speed.
Combine apple slices, lemon juice, and cinnamon sugar in a bowl. Fold into mixture. Spread mixture onto the prepared baking tray. Evenly spread with applesauce.
For the streusel combine flour, butter, sugar, ground almonds and vanilla sugar in a bowl. Crumble with your fingertips to obtain a sand like consistency. Sprinkle streusel over applesauce.
Bake in the preheated oven until streusel is lightly browned and cake is set, about 40 minutes.