Mix 1 cup of the rye flour, 2 cups of the all-purpose flour, the cereal, yeast, caraway seed, salt and onion powder in large bowl. Place 2 cups of the water, the vinegar, molasses, butter and chocolate in saucepan. Cook on low heat until butter is melted, stirring occasionally. Stir until chocolate is completely melted. Add to flour mixture. Beat with electric mixer on low speed just until blended. Beat on medium speed 3 minutes. Add remaining 2 cups rye flour and 3/4 cup all-purpose flour; stir until mixture forms a stiff dough.
Place dough on lightly floured board; knead 5 minutes or until smooth and elastic. Place in greased bowl; cover with clean towel. Let rise in warm place, free from draft, for 1 to 1-1/2 hours or until doubled in volume. Punch down dough; divide in half. Shape each half into ball. Place 1 ball in each of 2 greased 8-inch round baking pans or on 1 large greased baking sheet. Cover with clean towel; let rise 1 hour or until doubled in volume.
Preheat oven to 375°F. Bake, uncovered, 45 minutes or until loaves sound hollow when tapped. Meanwhile, blend cornstarch with remaining 1/4 cup water in small saucepan. Bring just to boil; brush over tops of loaves. Bake an additional 5 minutes or until glossy. Remove from pans; cool on wire racks. Cut each loaf into 16 slices to serve. Makes 2 loaves, 16 servings each.