Combine rye and all-purpose flour, set aside. In a large bowl, thoroughly mix 2 1/3 cups of the flour mixture, sugar, salt, cereal, caraway seeds, instant coffee granules, onion powder, fennel seed and yeast.
Combine water, vinegar, molasses, chocolate and butter in a saucepan. Warm over low heat to temperature of 1050 to 1150°F Gradually add to dry ingredients.
Beat at medium speed with electric mixer for 2 minutes, scraping bowl occasionally.
Add ½ cup flour mixture. Beat at high speed 2 minutes, scraping bowl occasionally. Stir in enough additional flour mixture to make soft dough.
Turn out on a lightly floured board. Cover dough with bowl. Let rest 15 minutes. Then knead until smooth and elastic, about 10 to 15 minutes. (Dough will be sticky) Place in greased bowl, turning to grease top. Cover and let rise in warm place, free from drafts, until doubled in size, about 1 hour.
Divide dough in half, form 2 loaves and place in 9-by-5 inch loaf pans. Or form into 2 round balls and place on greased baking sheet.
Preheat oven to 3500°F Bake 45 to 50 minutes or until bread sounds hollow when tapped on top.
Meanwhile, combine cornstarch and ½ cup cold water. Cook over medium heat, stirring constantly for 1 minute. As soon as bread is baked, brush cornstarch mixture over tops of loaves. Return bread to oven. Bake 2 to 3 minutes or until glaze is set. Remove from pans. Cool on wire racks.