1. Into large bowl, measure flour, butter or margarine, sour cream, salt, baking powder, 1/2 cup sugar, and 1 egg yolk (place egg white in small bowl and set aside). Stir ingredients until thoroughly blended and mixture holds together.
2. Shape dough into a ball; wrap with waxed paper or plastic wrap and refrigerate 2 to 3 hours until dough is firm enough to handle (or place dough in freezer 45 minutes).
3. In another small bowl, stir chopped walnuts, ground cinnamon, and 1/2 cup sugar. Set aside.
4. Preheat oven to 350°F. On lightly floured surface with floured rolling pin, roll one-third of dough 1/8 inch thick, keeping remaining dough refrigerated. Using 10-inch plate as guide, cut dough into 10-inch round. With pastry brush, spread dough round with some reserved egg white.
5. Cut dough round into 12 equal wedges. Place 1 teaspoon walnut mixture in center of each wedge. Starting at curved edge, roll up each wedge, jelly-roll fashion. Place cookies, point side down, about 1 1/2 inches apart, on ungreased large cookie sheet.
6. In cup, beat 1 egg and 1/2 teaspoon sugar. Brush cookies with some egg mixture. Repeat with remaining dough, egg white, and walnut mixture and reroll trimmings to make four rounds in all.
7. Bake cookies 15 to 20 minutes until golden brown. Remove cookies to wire racks to cool. When cookies are cold, store in tightly covered container.