1. In a small bowl, mix together yeast and warm water; set aside.

2. In the bowl of an electric mixer fitted with a paddle attachment, cream together butter and granulated sugar on medium speed, scraping down the sides of bowl with a spatula, as necessary. Add egg yolks, one at a time, mixing well after each addition and scraping down sides of bowl with a spatula as necessary.

3. Add vinegar and vanilla; mix to combine. Slowly add sour cream, mixing until well combined. Add yeast mixture.

4. In a large bowl, whisk together both flours and salt. Add to mixer and mix on low until well combined and dough begins to pull away from the sides of the bowl and just begins to form a ball.

5. Turn dough out onto a lightly floured work surface. Knead dough until smooth, about 5 minutes. Lightly flour the bottom of a large glass bowl; place dough in bowl and cover with a clean damp kitchen towel. Cover bowl tightly with plastic wrap; let stand until doubled in size, 3 to 4 hours, or refrigerate overnight.

6. Make the filling: Place all of the ingredients except egg and cream in a medium bowl and stir until well combined.

7. Punch down dough. Transfer to a floured work surface. Let stand for 5 minutes. Roll out to an 18-inch square (about 1/8 inch thick). Spread filling over dough, reserving 1/2 cup and leaving a 1-inch border. Brush edges with egg wash. Tightly roll dough like a jelly roll. Pinch seam to seal, and fold in half to bring ends together to form a U. Twist 2 or 3 times to "braid." Butter a 10-by-5-inch loaf pan, and line with parchment, leaving 1-inch overhangs; butter parchment. Transfer dough to pan. Brush top with egg wash.

8. Make the crumb topping: Combine confectioners' sugar, flour, and butter. Sprinkle topping and reserved 1/2 cup filling over cake.

9. Preheat oven to 375 degrees. Drape plastic wrap over dough. Let stand in a warm place until doubled in volume, about 1 hour.

10. Remove plastic wrap, and bake, rotating halfway through, until golden, about 55 minutes. Reduce oven temperature to 325 degrees. Bake until deeply golden, 20 to 30 minutes more (cover with foil if top gets too dark). Transfer pan to a wire rack. Let cake cool.