Mix the flour, baking powder and cornstarch and sift. Add the remaining ingredients and mix everything well. Pour the batter into a greased springform pan.
Clean the rhubarb and cut into small pieces. Cut the strawberries into small pieces and spread them on the dough with the rhubarb. Scatter the chopped hazelnuts on top.
Knead the flour, sugar, vanilla sugar and butter to make crumbles and spread on the fruit.