Mix flour, yeast, and butter thoroughly about 3 minutes on low speed. Add 3 whole eggs plus 3 yolks (reserve the remaining 3 whites for the filling) and sour cream. Mix on low to medium speed 20 minutes (do not underbeat). Place dough in a greased bowl, cover with plastic wrap, and refrigerate overnight.

Working with a small amount of dough at a time, roll out dough on a board or counter that has been liberally sprinkled with granulated sugar. Roll out very thin (dough will be satiny and easy to handle). Cut into triangles or squares about 2 inches on a side.