1. Combine 1 cup of water with the the cornmeal in a saucepan and cook over low heat until thick, stirring often (about 5 minutes). Remove the pan from heat and add the molasses, butter, salt, sugar, caraway seeds and cocoa powder. Stir to combine and set aside to cool to lukewarm.
2. Put the remaining 1/2 cup water in a large mixing bowl and add the yeast. If using active dry, stir and let the mixture "proof" for 10 minutes. If using instant, no waiting time is necessary.
3. Add the molasses mixture and mashed potatoes.
4. Stir in the pumpernickel and whole wheat flour. Mix and knead the dough at medium speed for 10 minutes.
5. After the dough is kneaded, it will look like this.
6. Transfer the dough to a greased bowl. Sprinkle the top with flour. Cover the bowl and let rise until the dough is not quite doubled; about 1 hour.
7. When the dough is risen, it will have expanded and you'll see cracks in the flour you sprinkled, like this.
8. Shape the dough into a loaf and place in a greased 8 1/2" by 4" loaf pan. Sprinkle the top with more flour or brush with a mixture of 1 egg white beaten with a tablespoon of water.
9. Slash the top in a decorative design, and bake in a preheated 375°F oven for 55 to 65 minutes, until the center measures 200°F when measured with an instant-read thermometer. Remove from the oven, tip out of the pan, and cool on a rack completely before slicing.