Step 1In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using a hand mixer, beat together the butter and flour. Beat in the sour cream, yeast and egg yolks, then beat in the salt just until combined. Gather the dough into a ball and wrap in plastic. Refrigerate for at least one hour.
Step 2While the dough is chilling, make the filling. In a small bowl, combine the cranberries, pistachios, granulated sugar and orange zest. In a separate bowl, beat together the egg whites and one-half cup powdered sugar until combined.
Step 3Heat the oven to 350 degrees. On a surface lightly dusted with powdered sugar, divide the dough into 5 equal pieces.
Step 4Work with one piece at a time, keeping the remaining pieces refrigerated. Roll the piece into a circle approximately 8 inches in diameter, working quickly so the dough does not warm. Cut the circle into 8 wedges (as with a pizza or pie), and brush over a light coating of the beaten egg white mixture.
Step 5Sprinkle over the cranberry-pistachio filling and, starting at the wide end, roll up each wedge to form a crescent. Place the crescents on a parchment-lined baking sheet. Repeat with the remaining pieces of dough, forming crescents. Space the cookies 1 to 2 inches apart, using additional baking sheets as needed to fit all the cookies.
Step 6Bake the cookies until set and lightly browned, about 15 minutes, rotating halfway through for even baking, then cool on a rack. Dust lightly with powdered sugar before serving.