Preheat the oven (175°C conventional (top/bottom heat))
2.
Peel 500g of the apples and cut them into (not too) small pieces. Put the lemon juice, cinnamon (or a cinnamon stick - remove it again after cooking) and sugar into a saucepan and let it simmer briefly, stirring as you go. Then set aside and let cool.
Dough:
3.
Beat the butter, sugar, vanilla sugar and salt until fluffy. Add the eggs one at a time.
4.
Mix the flour and baking powder and briefly stir into a smooth dough.
5.
Spread the dough onto a baking tray lined with baking paper.
Topping:
6.
Peel the remaining 1kg of apples and likewise cut them into not-too-small pieces. Then mix them with the cooked apples, taste, and add a little more cinnamon if needed. Spread the topping evenly over the dough base.
Streusel:
7.
Knead all the streusel ingredients together by hand. You should end up with a roughly crumbly mixture - typical streusel.
8.
Spread the streusel evenly over the apples and then bake in the oven for approx. 50 min. (middle rack).