Prepare the Dough: In a large bowl, whisk together the wheat flour, baking soda, salt, and vanilla sugar. In a separate, medium-sized bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the egg and the sour cream, stirring until all the ingredients are well combined and the sugar has largely dissolved. Gradually add the dry ingredients to the wet mixture, mixing with a spatula or your hands until a soft, slightly sticky dough forms. Be careful not to overwork the dough; just mix until it comes together. Cover the dough and let it rest for about 20-30 minutes.

Make the Filling: While the dough is resting, prepare the cottage cheese filling. In a medium bowl, combine the cottage cheese, flour, sugar, sour cream, and egg white. Using a fork or a hand mixer, stir the mixture until it is smooth and well-combined. If you prefer a completely smooth texture, you can use an immersion blender to mix the filling.

Assemble the Buns: Transfer the dough to a lightly floured surface. Roll the dough out to a uniform thickness of approximately 0.6-0.8 cm (¼ inch). Using an 8 cm (3-inch) round cutter or the rim of a small glass, cut out circles from the dough. Place a generous spoonful of the cottage cheese filling in the center of each circle. Fold the dough over the filling to create a half-moon shape and press the edges firmly to seal.

Bake: Preheat your oven to 200 ° C (392 ° F). Place the assembled buns on a parchment-lined baking sheet. In a small bowl, whisk the egg yolk with the milk to create an egg wash. Brush the tops of the buns with the egg wash for a beautiful golden-brown finish.

Finish: Bake the buns for 20 minutes, or until they are puffed up and golden brown. Let them cool on a wire rack for a few minutes before dusting them generously with powdered sugar. Serve the buns warm to best enjoy the creamy filling and tender dough.