1. Preheat the oven to 160°C (320°F). Line a 20cm springform tin* with baking parchment and grease it lightly. Peel, quarter, and core the apples, then cut them into slices. Set aside.

2. In a large bowl, mix the butter and sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition. Combine flour, ground hazelnuts, baking powder, and salt – add this mixture to the bowl and fold it in. Add about 2/3 of the sliced apples and all the blackberries, then fold them in. Transfer the batter to the prepared springform tin and smooth out the top. Arrange the remaining apple slices evenly all over the cake batter.

3. Place the butter flakes on top of the batter as well. Mix the brown sugar and cinnamon, then sprinkle the mixture over everything. Bake the cake in the preheated oven for about 50–55 minutes. After about 40 minutes, scatter the sliced hazelnuts over the cake and continue baking. The cake is done when a wooden skewer inserted into the center of the cake comes out clean. Take the cake out of the oven and let it cool down completely on a wire rack. If you like, dust the cake with some confectioner’s sugar before serving (optional).