Combine 1/2 cup flour, sugar, undissolved yeast and salt in a large bowl. Heat milk, water and butter until very warm (120° to 130° F). Gradually add to dry ingredients. Beat 2 minutes at medium speed of electric mixer, scraping bowl occasionally. Add eggs and 1 cup flour. Beat 2 minutes at high speed. Stir in remaining flour to make a stiff batter. Cover tightly with plastic wrap; refrigerate 2 to 24 hours.
Remove dough from refrigerator; divide into 2 equal pieces. On lightly floured surface, roll each piece to 20 x 10-inch rectangle. Spread rectangles evenly with Walnut Filling to within 1-1/2 inches of edges. Beginning at long side, roll up tightly as for jelly roll; pinch seams to seal. Form each into a coil, tucking end under. Place on greased baking sheets. Cover; let rise in warm, draft-free place until doubled in size, about 1 hour.
Bake at 350°F for 30 minutes or until done. Remove from sheets; cool on wire rack. Drizzle with Powdered Sugar Glaze, if desired.
Walnut Filling: In a saucepan, combine walnuts, 1/4 cup milk, 3 tablespoons sugar and cinnamon. Cook over medium low heat, stirring constantly, for 3 minutes or until thick. Remove from heat; let cool.
Beat egg white until soft peaks form in a medium bowl. Gradually add 1 cup sugar; beat until stiff. Fold into walnut mixture until well blended.
Powdered Sugar Glaze: In a bowl, combine powdered sugar and enough milk to reach desired consistency.