In a small bowl, dissolve yeast in water; set aside. In a large bowl, combine flour and salt; cut in butter until mixture resembles coarse crumbs. Add the egg yolks, sour cream, vanilla and yeast mixture. Mix well with hands (dough will not be sticky). , For filling, in a small bowl, beat egg whites on medium speed until soft peaks form. Gradually beat in sugar, 1 tablespoon at a time, on high until stiff peaks form. Beat in vanilla. Fold in walnuts. , Divide dough into eight portions. On a work surface heavily coated with confectioners' sugar, roll one portion into a 12-in. circle. Cut into 12 wedges. Spread 1 teaspoon filling over each wedge; tuck in edges of wide end and roll up toward narrow point. Repeat with remaining dough and filling, adding confectioners' sugar to work surface as needed. , Place on parchment-lined baking sheets. Bake at 375° for 12-14 minutes or until lightly browned. Remove to wire racks.