1. Preheat the oven to 200°C (400°F). Prepare and butter standard size tart pan with a removable base.
2. Prepare the crust by mixing soft butter together with sugar until well mixed, then add lightly beaten egg – add the flours, baking powder and vanilla sugar and mix to create a soft dough. Press the dough into the tart pan, evenly covering with a neat layer of dough both the bottom and sides of the tart pan.
3. Prebake the crust for 10 min. before adding the filling.
4. Make the filling by mixing together the sour cream, Crème fraîche, egg, sugar and vanilla sugar, then pour on prebaked crust. Evenly distribute the blueberries over the filling and bake until the filling has set and the crust is golden, about 20-25 minutes.