In the bowl of a heavy-duty electric mixer fitted with the dough hook, combine flour, cocoa powder, 1/3 cup sugar, 1 cup of water, yeast, and starter. Combine on low speed for 5 minutes. Let stand for 15 minutes.
Add salt and butter, and mix on medium speed until the dough develops a sheen, about 10 minutes. Add chopped chocolate. Continue to mix on medium speed until completely combined, 2 to 3 minutes. Place dough in a large lightly oiled bowl, and cover with plastic wrap. Allow to rise until doubled in size, about 3 hours. Fold dough into thirds (like an envelope), and cover with plastic wrap. Let rest for 30 minutes.
Butter 2 (6 3/4 by 3 by 2 1/4-inch) loaf pans and coat with remaining 2 tablespoons granulated sugar. Divide dough in half, then divide each half into 4 equal portions. Roll each piece of dough on work surface in a circular motion, forming a tight ball. Place 4 pieces of dough, smooth side up, in each loaf pan from 1 end to another. Cover tightly with plastic wrap. Allow to rise at room temperature until doubled in size, about 2 hours.
Meanwhile, adjust oven rack to center of the oven. Heat oven to 375 degrees F. Whisk egg yolk and cream to combine, and lightly brush risen loaves, and sprinkle with turbinado sugar. Place loaves in oven and reduce the oven temperature to 350 degrees F. Bake until loaves have a slightly hollow sound when tapped on the bottom, about 40 to 45 minutes.
Remove from oven and let rest on a cooling rack for 5 minutes before removing from pans. Finish cooling on wire rack.
In a medium bowl, dissolve yeast in water. Stir in flour until completely incorporated. Cover, and let stand at room temperature for 4 to 6 hours.
Yield: 3/4 cup
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