In the bowl of a food processor, combine the sifted flour, baking soda, vanilla sugar, and salt. Add the cold butter, cut into cubes, and pulse until the mixture resembles fine crumbs.Note: You can also make the dough using a kitchen knife instead of a food processor; the method is the same.
Then add the sour cream and a part of egg yolk, and continue processing just until the dough comes together into a ball. Do not overwork it. You should end up with a soft, pliable dough.
Wrap the dough in plactic wrap and and refrigerate for 30 minutes or overnight.
Preheat the oven to 185°C. At the same time remove the dough from the refrigerator.
Making Fillings
Prepare the dates by removing the pits and cutting each date lengthwise into two halves.
Shaping and Baking
Divide the dough into 2 parts. Roll out the dough to a thickness of 2 mm and cut out a 20 cm circle. Use a plate of a suitable diameter as a guide and trace around it with a knife. Or, like I do, use a pastry ring of the right size. Keep in mind that if the dough is rolled thicker, the cookies may unroll in the oven.
Cut each circle into 16 wedges.
Place half a date on the wide end of each triangle and roll the dough up around it, croissant-style.
Brush each crescent with egg yolk and sprinkle with sugar.
Arrange them on a baking tray lined with parchment paper and bake for 15–17 minutes. The crescents should puff slightly and turn golden brown.
Let them cool for a few minutes, then transfer to a wire rack to cool completely.