1. Preheat oven to 350°F (175°C). Grease and flour a 9-inch (23cm) springform pan or line a cake pan with parchment.

2. In a large bowl, combine 200g flour, 100g sugar, vanilla sugar, cold cubed butter, rolled oats, baking powder, and salt. Use a pastry cutter or fingertips to work mixture until it resembles coarse crumbs.

3. Press two-thirds of the crumb mixture into the bottom and slightly up the sides of the prepared pan. Chill while preparing the filling.

4. In a medium bowl, toss blueberries, 30g sugar, and cornstarch until evenly coated. Let sit 5 minutes.

5. Scatter white chocolate chips over the dough base.

6. Spread the blueberry mixture over the chocolate.

7. Break the remaining crumb mixture into small clumps and scatter evenly over the top.

8. Bake 35 to 40 minutes, or until the topping is golden and the filling is bubbling at the edges.

9. Cool 15 minutes before removing from pan. Serve warm or at room temperature with whipped cream or vanilla ice cream.