Step 1

Cut butter into flour in a bowl using 2 knives or a pastry blender until the mixture resembles coarse crumbs. Add sour cream and egg yolk; mix well. Shape the dough into a ball. Wrap in plastic wrap and refrigerate, 8 hours to overnight.

Step 2

Combine walnuts, sugar, and cinnamon in a small bowl.

Step 3

Preheat oven to 350 degrees F (175 degrees C). Lightly grease 2 baking sheets.

Step 4

Cut dough into 4 equal pieces. Roll each piece into a 1/8-inch thick circle on a lightly floured surface. Spread 1/4 of the walnut mixture on each circle; cut each circle into 12 wedge-shaped pieces with a pizza wheel.

Step 5

Shape each piece of dough into a crescent by rolling dough from the wide end of the wedge into the center. Transfer cookies carefully to the baking sheets.

Step 6

Bake in the preheated oven until golden, 20 to 25 minutes. Watch cookies carefully as they bake; their bottoms tend to brown faster than their tops.