Combine 3/4 cup flour and undissolved yeast in a large mixer bowl. Heat sour cream, butter and water until very warm (120° to 130°F). Gradually add to flour mixture. Beat 2 minutes at medium speed of electric mixer, scraping bowl occasionally. Add egg yolks and 1/2 cup flour. Beat 2 minutes at high speed, scraping bowl occasionally. Stir in enough remaining flour to make a stiff batter; portion into 3 parts. Cover each portion tightly with plastic wrap; refrigerate 2 to 24 hours.
For Chocolate-Nut Filling, beat egg whites in medium bowl until soft peaks form. Gradually add sugar and vanilla. Continue beating until stiff, but not dry. Fold in in walnuts and chocolate chips.
Lightly dust work surface with 1/2 cup sugar. Roll each piece of dough to a 12-inch circle; cut each into 8 wedges. Spread about 1 tablespoon Chocolate-Nut Filling on wide end of each wedge. Roll up from wide ends to points; curve ends to make crescents. Place on greased or parchment-lined baking sheets with points facing down. (Kifle will unroll when baking if points are not facing down.)
Bake in preheated 375°F oven for 15 to 20 minutes or until golden brown. Remove from sheets; cool on wire racks. Sprinkle with powdered sugar.