1. To prepare the dough, combine first 3 ingredients in a large bowl, and let stand for 5 minutes or until bubbles form on the surface. Add butter, vanilla, salt, and eggs; stir. Weigh or lightly spoon 22.5 ounces flour (about 5 cups) into dry measuring cups; level with a knife. Add flour to egg mixture; stir until combined. Turn dough out onto a lightly floured surface. Knead 4 minutes; add enough of remaining 1/4 cup flour to prevent the dough from sticking. Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place, free from drafts, 1 1/2 hours or until doubled in size.
2. To prepare the filling, combine 1/3 cup sugar and cocoa in a small bowl, stirring well. Combine egg white and 2 teaspoons milk in a small bowl.
3. Divide dough into two equal portions. Working with one portion at a time (cover remaining dough to prevent drying), roll dough into a 12 x 9-inch rectangle. Melt 1/4 cup butter; brush 1 tablespoon over dough. Sprinkle evenly with half of cocoa mixture, leaving a 1/4-inch border. Sprinkle dough evenly with 2 ounces chopped chocolate. Drizzle with an additional 1 tablespoon butter. Brush far edge with milk mixture. Loosely roll up, starting with a long edge; pinch seam to seal, and tuck ends under. Place dough, seam side down, on a baking sheet lined with parchment paper. Repeat procedure with remaining dough, butter, cocoa mixture, chocolate, and egg white mixture. Reserve remaining milk mixture. Cover dough. Let rise in a warm place 1 hour or until doubled in size.
4. Preheat oven to 325°.
5. Brush the dough with remaining milk mixture. Sprinkle each loaf with 2 1/4 teaspoons turbinado sugar. Bake at 325° for 40 minutes or until browned. Cool loaves completely on a wire rack. Cut each loaf into 12 slices.