Chocolate Bread Starter: Dissolve yeast in water for 10 minutes; stir in flour until completely mixed; cover loosely and leave to proof at room temperature 6 hours.
Chocolate Yeast Bread: Stir and thoroughly mix flour, cocoa, 1/3 cup sugar, yeast, water and 1/4 cup Chocolate Bread Starter (save the remainder for another use); let rest 15 minutes.
Scrape dough onto floured work surface and knead in salt and butter; knead until smooth and elastic, about 10 minutes; knead in chopped chocolate thoroughly.
Place dough in lightly oiled bowl and cover with plastic wrap; let rise until doubled, about 1 1/2 hours; leaving dough in bowl, fold dough into thirds as if folding a letter for an envelope, recover with plastic wrap; let rest 30 minutes.
Butter two 7 x3 x2 loaf pans and coat with remaining 2 T granulated sugar; divide dough in half and then divide each half into 4 even pieces; roll and form each piece of dough into a tight ball; place four pieces, smooth side up, side-by-side in each loaf pan; cover tightly with plastic wrap and let rise at room temperature until doubled, about 2 hours.
Preheat oven to 375°; combine egg yolk and cream and brush on loaves; sprinkle with turbinado sugar; place in oven, reduce temperature to 350° and bake for 40-45 minutes or until loaves have a slightly hollow sound when tapped on the bottom.
Let rest in loaf pans for 5 minutes before removing to wire rack.