The Best Butterhorn Cookies You'll Ever Make

In a large mixing bowl or food processor, combine the flour, sugar, salt, and baking powder. Cut in cold butter until it looks like coarse crumbs. In a medium bowl, whisk together the sour cream and egg yolks. Add the wet ingredients to the flour mixture and blend just until clumps begin to form.
Press the dough together and shape dough into two equal balls. Flatten into discs, wrap tightly in plastic wrap, and refrigerate for at least 2 hours or overnight. Chilling lets the butter firm up and makes the dough easier to handle.
In a small bowl, combine sugar, ground walnuts, cinnamon, and 1 egg white. (half of the egg whites you saved) Mix until it resembles damp sand.
On a lightly floured work surface, roll one half of a piece of dough into a 9-inch circle. Use a pizza cutter to cut into 12 equal pieces.
Spoon a small amount of filling near the wide end of each wedge. Roll into crescents, place on ungreased cookie sheet. Repeat with the remaining dough pieces.
Brush each cookie with an egg wash made by combining the remaining egg white with 1 tablespoon warm water. Sprinkle with course sugar and bake at 350°F (175°C) for 18–22 minutes, until lightly golden brown. Transfer to a wire rack to cool. If desired, dust with powdered sugar before serving.