Makes 2 loaves
Sprinkle yeast and sugar over 1/4 cup water; stir to dissolve. Let stand until foamy, about 10 minutes.
Melt chocolate and margarine with 1 1/4 cups water in large bowl set over gently simmering water. Stir until smooth. Remove from over water. Blend in molasses, vinegar and salt. Mix in cereal. Let cool.
Grease large bowl. Blend yeast into cereal mixture. Gradually stir in 2 cups all-purpose flour and rye flour. Turn dough out onto lightly floured surface and knead until smooth and elastic, about 10 minutes, kneading in up to 3/4 cup more all-purpose flour if necessary to form workable dough. Add dough to prepared bowl, turning to coat entire surface. cover and let rise in warm draft-free area until doubled in volume, about 2 hours.
Grease two 3x2x7-inch loaf pans. Punch dough down. Turn out onto lightly floured surface and let rest 3 minutes. Knead 3 minutes. Divide dough in half. Roll each into 7x8-inch rectangle. Starting with long side, roll dough up into cylinder. Tuck ends under and pinch seam to seal. Place seam side down in prepared pans. Cover and let rise in warm draft-free area until doubled, about 1 1/2 hours.
Preheat oven to 375 F. Bake until loaves sound hollow when tapped on bottom, about 45 minutes. Remove bread from pans. Let cool completely on rack before serving.