Combine sour cream, milk, 3/4 cup sugar and salt in a medium-size saucepan. Heat slowly, stirring constantly, just until mixture begins to bubble and sugar dissolves; cool in large bowl.

Sprinkle yeast and the 1 teaspoon sugar into very warm water in a 1-cup measure. (Very warm water should feel comfortably warm when dropped on wrist.) Stir to dissolve. Let stand until bubbly, about 10 minutes.

Stir yeast into cooled sour cream mixture; beat in 3 cups of the flour until very smooth. Cover bowl with clean towel; let rise in a warm place, away from drafts, 1 hour or until doubled in volume; beat mixture down.

Beat egg yolks until well blended in a medium-size bowl; beat in very soft butter until smooth. Beat egg/butter mixture and enough of the remaining flour into yeast mixture to make a soft dough.

Turn out onto lightly floured surface and knead 10 minutes or until dough is smooth and elastic. Place in a large buttered bowl; turn to bring buttered-side up. Cover; let rise in a warm place, away from drafts, 1 hour or until doubled in volume.

Punch dough down; knead a few times. Let rest 5 minutes. Divide dough in half; roll one-half on a lightly floured surface to a 26 x 10 inch rectangle. Spread half the walnut filling over dough, leaving 1/4 inch margins.

Roll up jelly roll fashion, starting with one of the short ends. Pinch to seal seam; turn ends under, pinching to enclose filling. Place, seam-side down, on a greased cookie sheet. Repeat with the remaining dough and filling to make a second cake. Cover; let rise in a warm place, away from drafts, 1 hour or until almost doubled in volume.

Bake in a moderate 350°F oven for 35 minutes or until cakes are golden. Cool on wire racks. Brush cakes with warmed honey; sprinkle with reserved chopped walnuts, if you wish.

Finely chop or grind walnuts; make sure that the nuts are very fine, so that the cake will cut neatly. Reserve 1/4 cup for garnish, if you wish. Beat egg whites until frothy. Add sugar and nuts. Mix well.