Balthazar Chocolate Bread

2½ cups bread flour
¼ cup dutch-process cocoa powder
⅓ cup plus 2 T granulated sugar
2 tsp. active dry yeast
¼ cup bread starter
1¼ teaspoon salt
2 T unsalted butter room temperature
3 ounces bittersweet chocolate cut into ¼ -inch pieces
vegetable oil for bowl
1 large egg yolk lightly beaten
1 T heavy cream
2 T turbinado sugar

Starter:
¼ teaspoon active dry yeast
1 cup warm water
1 cup + 3 T all-purpose flour

For 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 ¼ cup flour to feed starter.

For the chocolate bread:
In the bowl of a heavy duty electric mixer fitted with the dough hook, combine bread, flour, cocoa powder, ⅓ 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 6¾ x 3 x 2¼-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. 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.