Preheat oven to 350°F (175°C). Grease and line a 9-inch springform pan.

Prepare the filling: In a medium bowl, toss the chopped rhubarb, raspberries, 100g sugar, and cornstarch until evenly coated. Set aside.

Prepare the crumble: In a separate small bowl, combine the topping flour, brown sugar, and cinnamon. Cut in the cold diced butter using your fingers or a pastry blender until coarse crumbs form. Chill until needed.

Prepare the cake batter: In a large bowl, cream together the softened butter and 200g sugar until light and fluffy. Beat in the eggs one at a time, followed by the vanilla extract.

In a separate bowl, whisk together the 250g flour, baking powder, and salt. Alternate adding the dry mixture and the buttermilk to the wet ingredients, beginning and ending with the dry, mixing just until combined. Do not overmix.

Assembly: Pour half of the cake batter into the prepared pan. Spoon half of the raspberry rhubarb filling evenly over the batter. Pour the remaining batter over the filling (it may not cover completely). Top with the remaining fruit filling, and finally sprinkle the chilled crumble mixture evenly over the top.

Bake for 45 to 50 minutes, or until a toothpick inserted into the cake layer (avoiding the fruit) comes out clean. Let cool in the pan for 15 minutes before releasing the sides and transferring to a wire rack to cool completely.