Place flour and baking soda in a large bowl; stir to combine. Cut butter into flour mixture with pastry blender or two knives until mixture forms pea-sized pieces.
Reserve 1 egg yolk in cup; cover with plastic wrap. Add remaining yolk and sour cream to flour mixture; toss with a fork until mixture holds together. Form dough into a ball; wrap in plastic wrap and refrigerate 2 hours or until firm.
Meanwhile, for filling, combine dried fruit with Bourbon or Brandy and vanilla extract in a medium bowl. Set aside and let sit 90 minutes. Stir in ground walnuts, sugar and ground cinnamon. Let sit 30 minutes longer.
Beat egg whites in a clean large bowl with electric mixer at high speed until stiff peaks form. (After beaters are lifted from egg white mixture, stiff peaks should remain on the surface and when bowl is tilted, mixture will not slide around.
Gently fold in fruit mixture with rubber spatula by gently cutting down to the bottom of the bowl, scraping up the side of the bowl, then folding over the of the mixture. Repeat until fruit mixture is evenly incorporated into the whites.
Grease and lightly flour 9-inch square baking pan. Divide dough into 3 equal pieces. Shape each piece into a 3-inch square about 1-inch thick. Place each square between two pieces of well-floured waxed paper. Roll dough into a 9-inch squares. (You may have to lift the paper and dust dough with flour occasionally to prevent sticking.)
Remove top sheet of waxed paper from one dough square. Invert dough into bottom of baking pan. (You may have to trim dough to fit pan.) Remove second sheet of waxed paper.
Add water to reserved egg yolk; beat lightly with fork until foamy. Brush dough with egg yolk mixture. Spread half of fruit mixture on dough. Repeat procedure with second dough square, egg mixture and remaining fruit mixture. Cover with third dough square; turn edges under for a neat appearance.
Preheat oven to 350 degrees. Score top of dough into diamonds by making lengthwise diagonal cuts, just through top layers of dough, every 1 1/2 inches, with sharp knife. Make crosswise horizontal cuts, just through top layers of dough, to form diamond shapes.
Brush dough with remaining egg yolk mixture. Bake 45 minutes or until browned. Remove pan to wire rack; cool completely. Cut bars all the way through with serrated knife; carefully lift bars out of pan with thin flexible spatula.
Store tightly covered in single layer. Makes about 2 1/2 dozen bars.