Preheat oven to 350 degrees. Grease 2 baking sheets and sprinkle with cornmeal and set aside. Grease a large bowl and set aside.
In a mixing bowl, dissolve yeast in warm water. Stir in the molasses, butter, rye flour, cocoa, salt, 2 cups flour, and raisins if using until smooth. Mix in enough ramining flour to form a stiff dough.
Turn onto a floured surface; knead until smooth and elastic, about 6-8 minutes. Place in prepared bowl, turning once to grease top. Cover and let rise in a warm place until doubled in size, about 1 1/2 hours.
Punch dough down. Turn onto a lightly floured surface; divide in half. Shape into loaves, about 10" long. Place onto prepared pans. Cover and let rise until doubled, about 1 hour.
Bake for 35-40 minutes or until bread tests done. Remove from pans to wire rack to cool.