1. Dough: Combine flour and salt in medium-sized bowl. Add yeast and butter and cut butter into flour using a pastry blender or 2 knives like you would for a pie crust until mixture resembles small crumbs.
2. Combine sour cream, yolks and vanilla in small bowl; add to flour mixture and combine to form dough.
3. Filling: Beat egg whites and vanilla extract until foamy; gradually add in the sugar and beat until stiff. Fold in nuts. Chill.
4. Preheat oven to 325. Cut dough into 8 balls. Roll out each ball into a circle on a board with rolling pin that is sprinkled with Powdered sugar (instead of flour like you normally do a pie) until it is thin like a pie crust would be. Cut each circle into 8 pie-shaped wedges. Place about 2 tsp. of filling on each wedge and spread to cover almost to edge. (Be generous with filling but if too full, the filling will ooze out too much on baking sheet.)
Roll up dough over filling like a crescent roll and turn ends around slightly to make crescent horn shape.
5. Place cookies about 1 ½ inches apart on parchment-lined or greased flat cookie sheets. Bake at 325 for about 40 minutes or until lightly browned on bottom of cookies. Cool on wire rack.
6. Sprinkle cookies with more powdered sugar. These cookies freeze very well.