In a large bowl, beat margarine and sour cream until blended. Add egg yolk; mix well. Combine flour and salt; add to margarine mixture and mix well (dough will be sticky). Cover and refrigerate for 4 hours or overnight., Preheat oven to 350°. Divide dough into eighths. On a well-floured surface, roll each portion into a 10-in. circle. Cut each circle into 8 triangles. Combine the walnuts, sugar and cinnamon; sprinkle over each triangle. , Roll up triangles from the wide ends and place point side down 1 in. apart on parchment-lined baking sheets. Bake until lightly browned, 20-22 minutes. Remove to wire racks to cool. Dust with confectioners' sugar. Store in an airtight container.