In a large mixing bowl, combine flour and butter, cutting together with either two knives or a pastry cutter.
In a separate mixing bowl, combine sour cream, yeast, and sugar. Mix until well combined and let sit while yeast dissolves.
Add yeast mixture into the flour mixture along with the vanilla and egg yolks. Mix dough together by hand until it is well combined. Split dough in half and wrap up in plastic wrap. Refrigerate.
Right before dough comes out of refrigerator, combine ingredients for filling - egg whites, walnuts, sugar, lemon juice. Set aside.
On a large cutting board, sprinkle 1/3 cup flour and 1/3 cup sugar. Roll out dough into a thin circle before cutting into 8 triangles.
Place 1 tsp of filling on each triangle and roll up starting on the wider end. Place on a lined cookie sheet with the open end down and bend the cookie a bit so it looks like a crescent. Repeat with process for all of the triangles from both things of dough.
Bake at 350 degrees for 20-25 minutes. Sprinkle powdered sugar on top and serve.