Cream the Butter and Sugar: In a large mixing bowl, cream together the softened butter and sugar until the mixture is light and fluffy. This step is important for incorporating air into the dough, which helps to achieve a tender texture.

Add Sour Cream and Egg Yolk: Mix in the sour cream and egg yolk until well combined. The dough should be smooth and slightly sticky.

Mix in Dry Ingredients: Gradually add the flour and baking powder to the wet mixture. Start by adding half of the flour, mixing well, and then gradually add the rest until the dough comes together. It should be soft but not too sticky. If necessary, add a little more flour to reach the right consistency.

Chill the Dough: Form the dough into a ball, wrap it in plastic wrap, and refrigerate for at least 30 minutes. Chilling the dough makes it easier to handle and roll out.

Whisk the Egg White: In a small bowl, whisk the egg white with a pinch of salt until it’s frothy. This will help to give the filling a light texture.

Combine with Sugar and Nuts: Stir in the sugar, vanilla sugar, and chopped nuts until everything is well combined. The filling should be slightly sticky and hold together when pressed.

Preheat the Oven: Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper to prevent the pastries from sticking.

Roll Out the Dough: On a lightly floured surface, roll out the chilled dough to about 1/4 inch (0.6 cm) thickness. Use a round cookie cutter or a glass to cut out circles from the dough.

Add Filling: Place a small spoonful of the nut filling in the center of each dough circle.

Seal the Pastries: Fold the dough over the filling to form a half-moon shape. Press the edges together to seal the pastry. You can crimp the edges with a fork for a decorative touch.

Bake: Place the filled pastries on the prepared baking sheet. Bake in the preheated oven for 15-20 minutes, or until the edges are lightly golden. Keep an eye on them to avoid overbaking.

Cool the Pastries: Once baked, remove the pastries from the oven and let them cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.

Optional Topping: For an extra touch, you can dust the cooled pastries with powdered sugar or drizzle them with a simple glaze made from powdered sugar and milk.

Serve: Enjoy these delightful pastries with a cup of tea or coffee. They also make a wonderful addition to a dessert platter or a sweet treat to share with friends and family.