1. In a small bowl, stir the yeast into the warm milk and let it sit for 5 minutes until foamy.

2. In a large bowl, whisk the flour and salt. Cut in the cold butter with a pastry cutter or your fingers until the mixture resembles coarse crumbs.

3. Whisk the sour cream, egg yolks, 1/4 cup sugar, and the yeast mixture together. Stir into the flour mixture until a soft dough forms.

4. Divide the dough into 8 equal pieces, shape each into a disk, wrap, and refrigerate overnight.

5. For the filling, stir the ground walnuts, 1 cup sugar, egg whites, and vanilla together until it forms a thick, spreadable paste.

6. Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper.

7. On a floured surface, roll one disk into a thin 9-inch circle. Spread with a thin layer of filling and cut into 8 wedges.

8. Starting at the wide end, roll each wedge up toward the point. Place on the baking sheet with the point tucked underneath and curve slightly into a crescent.

9. Bake for 15 to 20 minutes, until lightly golden on the bottom. Cool on a wire rack and dust with powdered sugar.