1. Dough. Heat oven to 170 degrees for 10 minutes; turn off. Heat milk in a small pot to approximately 110 degrees . Stir in 1 tbsp of the sugar. Gently stir in yeast; let stand 5 minutes.
2. In a large bowl, stir together flour, remaining 3 tbsp sugar and the salt. Stir in softened butter, beaten egg plus yolk, and sour cream. Make a well in the center and pour in yeast mixture; mix well. Knead dough 10 minutes, until soft. Dough should spring back slowly when touched. Divide into 2 pieces and place in separate greased bowls. Cover each with plastic wrap and a clean kitchen towel; set aside in warm oven to rise for 1 hour.
3. Filling. Meanwhile, heat half-and-half in a small pot. Pour over ground walnuts and cubed butter in a medium bowl. Stir in honey, 3 tbsp of the sugar, the orange zest, cinnamon, vanilla and beaten egg yolk. In a separate bowl, whisk 2 of the egg whites with remaining 1 tbsp sugar to stiff peaks with a hand mixer. Fold into walnut mixture.
4. Roll out one mound of dough on a lightly floured surface to a 16 x 10-inch oval. Spread a heaping cup of the filling on oval, leaving a 2-inch border on one of the short ends and a 1-inch border around remaining edges. Sprinkle 1/3 cup of the raisins over filling. Roll dough, starting from short end with 1-inch border and finishing at 2-inch border. Pinch to seal seam and ends. Place on a greased baking sheet seam side down; cover loosely with plastic wrap and kitchen towel. Repeat with second mound of dough and another heaping cup of the filling and the raisins (there will be filling leftover). Place on a separate baking sheet. Allow loaves to rise for 1 hour. Heat oven to 350 degrees .
5. Beat remaining egg white. Brush onto each loaf. Place on middle racks of oven and bake at 350 degrees for 25 minutes. Reduce heat to 325 degrees and rotate loaves. Bake another 20 minutes, until golden brown. Cool before slicing.