Place the flour in a large bowl, add sugar, butter, baking powder, yolks and sour cream. Knead the dough, then wrap it in a plastic wrap and put in the fridge to cool for about 1 hour.
Grease and flour your baking pan. I used a 35 x 25 cm pan. Roll the dough to fit the bottom of the pan may do sides as well and prick the bottom with fork. Place the pan in a preheated oven to 350 F and bake for about 20-25 minutes until golden brown. After baking, set aside to cool.
Place the cut rhubarb into a pot, add sugar and simmer over low heat until the rhubarb softens and releases the juice. Then add strawberries into the pot and mix everything together. Pour the rhubarb strawberry mass into a strainer and leave to strain (the juice that has run down can be used to prepare a delicious compote, all you need to do is add water). Then set aside the mass to cool.
Now prepare the crumble by adding softened butter, sugar and vanilla sugar to the flour. Mix everything with your fingers until all the ingredients are combined.
Place the fruit mass on the previously baked dough and spread it evenly. Sprinkle the top with crumble (you may use a grater) and put it back in the preheated oven for about 40-45 minutes at 350 F until the crumble turns golden brown. Let it cool completely. You may sprinkle with powdered sugar. Enjoy!