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.

Prepare filling: Mix dates and nuts with lightly beaten egg whites. Then place a small dab of nut mixture in the center of each piece. Roll up squares on the diagonal; roll up triangles from the longest side to the tip. Place on ungreased cookie sheets and bake at 350º 8 to 10 minutes, until slightly browned. Cool on racks. Cookies may be kept in a tightly covered container for about a week or may be frozen, well wrapped, for at least a month.