Makes 1 cake in a 23cm Springform
Preheat the oven to 170C fan.
Start by peeling and coring the apples, then cut them into 1 cm chunks. Set aside.
Put the butter, sugar and vanilla sugar in a bowl and whisk until pale and fluffy. Add the eggs, one at a time, and beat between additions. Add the Greek yoghurt and mix again briefly.
In a separate bowl, mix the flour, baking powder, baking soda, cinnamon and salt. Fold the dry ingredients through the wet ingredients, making sure not to overmix. Pour the sparkling water in the dough while whisking.
Toss the apples in 1 tablespoon of flour and fold through the batter.
Pour the batter into a 23cm Springform lined with baking paper.
In a medium bowl, combine the flaked almonds, brown sugar and cinnamon, and sprinkle over the cake.
Bake the cake for 50 minutes. Put on a wire rack to cool completely before removing from the Springform and serving.