Prepare the dough base: In a mixing bowl, combine flour, baking powder, sugar, and salt. Add the soft butter, egg yolk, and sour cream. Mix until a soft, smooth dough forms. Press the dough evenly into the base of a 24cm (9.5-inch) springform pan lined with parchment paper.

Pre-bake the crust: Bake the crust at 180°C (356°F) for 10 minutes. Remove and let it cool slightly while preparing the filling.

Make the cheese filling: In a bowl, beat the cream cheese with vanilla sugar, egg yolks, and cornstarch until smooth and creamy. In a separate bowl, whip the egg whites until soft peaks form, gradually adding the sugar until stiff and glossy. Gently fold the meringue into the cream cheese mixture.

Add the berries: Rinse and pat dry the blueberries and currants. Gently fold them into the cream mixture.

Pour and spread: Pour the mixture over the pre-baked crust and smooth the top evenly.

Prepare the crumble: In a bowl, rub together flour, sugar, and butter using your fingers until it resembles coarse crumbs.

Top and bake: Sprinkle the crumble mixture evenly over the cheesecake. Bake at 180°C (356°F) for about 50 minutes or until golden brown and set.

Cool and serve: Let the cake cool completely in the pan, then refrigerate for at least 2 hours before serving for the best texture.