Cut butter into flour until evenly distributed. Small chunks of butter are ok.

Mix egg yolk and sour cream together, then add to flour mixture, stirring just until dough comes together.

Divide dough into 3 equal parts, wrap in plastic wrap and refrigerate for at least 30 minutes or overnight.

Combine sugar, cinnamon and walnuts; set aside.

Preheat oven to 375°F. Line cookie sheets with parchment paper.

Roll one portion of dough into 12-inch circle. Sprinkle with ⅓ of the sugar mix. Cut circle into 16 even wedges and roll up firmly starting with the wide end.

Place on cookie sheet and space 2 inches apart. Bake for 15-20 minutes or until delicately browned. Repeat with remaining 2 portions of dough.