1. For Dough:
2. Crumble yeast into flour. Cut up butter into tablespoon size pieces. Cut butter into flour using fork. Add yolks, vanilla, and sour cream. Mix well with hands.
3. Divide into 6 balls. Refrigerate.
4. For Filling:
5. Beat egg whites. Add sugar slowly while mixing. Add vanilla. Mix in nuts by hand.
6. Roll out dough into a circle. Cut into 12 even wedges. Spread some of the filling onto each wedge. Roll each wedge from widest end to the point (similar to a croissant. Place on ungreased cookie sheet and bake at 325.F for 10 minutes.
7. Transfer to wire rack to cool. Sift powdered sugar over butter horns when cool.