Balthazar Chocolate Bread

Ingredients for bread:
2 1/2 cups bread flour
1/4 cup dutch-process cocoa powder
1/3 cup plus 2 T granulated sugar
2 teaspoon active dry yeast
1/4 cup bread starter
1 1/4 teaspoon salt
2 T unsalted butter, room temperature
3 ounces bittersweet chocolate, cut into 1/4-inch pieces
vegetable oil for bowl
1 large egg yolk, lightly beaten
1 T heavy cream
2 T turbinado sugar

Ingredients for starter:
1/4 teaspoon active dry yeast
1 cup warm water
1 cup plus 3 T all purpose flour

Preparing the starter:
In a medium bowl, dissolve yeast in 1 cup warm water. Stir in flour until completely incorporated. Cover; proof at room temperature 4 to 6 hours. Store in a plastic container, refrigerated, with plastic wrap covering but not touching the starter. Once a week, stir 1/4 cup flour to feed starter.

Preparing the chocolate bread:
In the bowl of a heavy duty electric mixer fitted with the dough hook, combine bread, flour, cocoa powder, 1/3 cup sugar, 1 cup water, yeast, and bread starter. Combine on low speed for 5 minutes. Let stand 15 minutes.

Add salt and butter, and mix on medium speed until the dough develops a sheen, about 10 minutes. Add the chocolate. Continue mixing on medium speed until completely combined, 2 to 3 minutes. Lightly oil a large bowl; place dough in bowl; cover with plastic wrap. Allow dough to rise until doubled in size, about 3 hours.

Turn out dough and fold into thirds, and cover with plastic wrap. Let the dough rest 30 minutes. Butter two 6 3/4 x 3 x 2 1/4-inch loaf pans; coat with remaining 2 tablespoons granulated sugar. Divide dough in half; divide each half into four equal portions. Roll each piece of dough on a clean work surface in a circular motion, forming a tight ball. Place four pieces of dough, smooth sides up, in each loaf pan. Cover pans tightly with plastic wrap. Allow the dough to rise until doubled in size, about 2 hours.

Position oven rack in center of oven; heat to 375 degrees. In a bowl, combine egg yolk and cream. Lightly brush risen loaves with egg wash and sprinkle with turbinado sugar. Reduce oven to 350 degrees F. Bake until loaves have a slightly hollow sound when you tap tops with your finger, 40-45 minutes; turn out loaves. Return loaves to wire rack; let cool completely. Wrap tightly in plastic and store up to 3 days or freeze up to 1 month.