Heat the oven to 175 C / 350 F. Line two large sheet pans with parchment paper.

Combine the flour, baking powder, salt, and vanilla sugar in a large mixing bowl and whisk briefly.

Add the cold butter cubes and rub them into the flour with your fingertips until the mixture looks like coarse breadcrumbs.

Add the egg yolks and sour cream, then mix until a soft, smooth dough comes together. It should not be sticky. If it is, add flour one tablespoon at a time.

Set the dough aside while you make the filling. Do not refrigerate.

Pulse the walnuts in a food processor until finely ground but not pasty, about 15 to 20 short pulses.

Transfer to a bowl and stir in the sugar and cinnamon.

Add warm milk one tablespoon at a time, stirring after each addition, until the filling holds together in a soft paste. You may not need all 3 tablespoons.

Pinch off a piece of dough roughly the size of a large walnut, about 20 g.

On a lightly floured surface, roll it into a circle about 8 cm wide and 3 mm thick.

Place a small teaspoon of walnut filling along the lower third of the circle.

Roll the dough up over the filling, away from you, into a log shape, then curve the ends gently downward to form a crescent.

Place on the prepared sheet pan. Repeat with the remaining dough and filling, spacing cookies about 2 cm apart.

Bake for 13 to 15 minutes, until the cookies are set and the bottoms are just barely pale gold. The tops should stay cream-colored.

Remove from the oven and let cool on the pan for 3 minutes.

Sift a generous layer of powdered sugar over the warm cookies. Let cool completely, then sift a second layer of powdered sugar over them before serving.