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 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.
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.
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.
Preheat oven to 32
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.
