Prepare your cake tin by greasing the sides with butter and covering the base with baking parchment. Preheat the oven to 180°C/ 356°F top and bottom heat.
Sieve the flour, baking powder, salt, and corn flour/ starch into a bowl and set aside.
Peel the apples, core them and cut them into slices. Cover them in lemon juice so they don’t brown.
Make the streusel topping: In a medium-sized bowl, combine the sugar, vanilla sugar, cinnamon and butter. Using a rubber spatula, stir in the flour. The streusel topping should be thick and crumbly. Place in the fridge until needed.
Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together at high speed until smooth and creamy, which should take around 2 minutes. Add the eggs and mix well for a further minute. Add the lemon peel. Pour the flour mixture into the batter and mix at a low speed until everything is combined.
Spoon or pour the apple cake batter into your baking tin. Spread out the batter to form an even surface with the rubber spatula if needed.
Place the apple slices in a circular arrangement on top of the base.
Now remove the streusel topping from the fridge. With your fingers crumble the dough over the cake.
Place in the oven and bake for approximately 35-45 minutes. The exact baking time will depend on your oven. To check if your cake is done, prick a skewer into the middle of the cake - if it comes out clean it is done.
Keep the cake at room temperature until it is ready to serve. If you like you can dust the cake with some icing/ confectioners’ sugar before serving.
You can serve this cake warm or cold. It goes well with a dollop of freshly whipped cream.