1. In a bowl, dissolve yeast in 1/2 cup warm water (90° to 105°). Let sit 5 to 10 minutes.
2. Meanwhile, in a small saucepan over medium-low heat, combine molasses, butter, vinegar, and chocolate. Cook until butter and chocolate are melted. (Or, put in a glass or porcelain bowl and microwave, covered, in 20-second increments at full power until melted.) Pour mixture into a medium bowl and add 2 cups water. Set aside.
3. In the bowl of a stand mixer, combine whole-wheat flour, rye flour, cocoa powder, caraway seeds, salt, and espresso powder. Attach dough hook and, with mixer on low, mix in yeast and molasses mixtures. Add 1 1/2 cups bread flour and mix to combine. Add more bread flour, 1/4 cup at a time, until dough pulls away from the inside of bowl. Knead (with mixer on low) until smooth and elastic, adding more flour as necessary to keep dough from sticking too much. When ready, the dough should feel a bit like an earlobe when you pinch it.
4. Put dough in a large, oiled bowl. Cover loosely with a clean towel or plastic wrap and let rise until doubled in bulk, about 1 hour.
5. Punch down and divide in two equal pieces. Knead and shape into round loaves about 9 in. in diameter. Sprinkle a large baking sheet with cornmeal and place loaves, smooth side up, on sheet. Cover and let rise until doubled in bulk, about 1 hour.
6. Preheat oven to 350°. Brush loaves with egg and use a razor or serrated knife to slash 1/4- to 1/2-in.-deep crosses on the tops. Bake 45 minutes to 1 hour (they're done if they sound hollow when tapped on the bottom) and cool completely on wire racks before cutting.
Note: Nutritional analysis is per 2-oz. serving.