Mix flours with salt and butter, add sour cream and egg, knead it to a soft dough, roll it into a plastic foil and let it rest in the fridge for a couple of hours or even better for a complete night.
For the filling bring milk to boil, add poppy-seed and semolina than finally sugar and vanilla sugar. Stir it well and let it cool down. On a lightly floured surface roll out the dough to 0,5 cm thick and cut it into 6 x 6 cm rectangular. Divide filling among the rectangles than roll them up, push them a bit so the dough sticks together and place them on a baking pan lined with parchment paper. Important is that the folding is on the bottom so they won’t open while baking and leave some space between them because they will grow a bit. Preheat the oven to 200C (390F) degrees and bake them until golden brown. Mix confectioner’s sugar and vanilla sugar and while the cookies are still hot, roll them into the mixture.