Cover bottom of large casserole with oil and heat- Brown pieces about 15 minutes. Add onion, garlic, carrots, green pepper, bay leaves, salt, red wine and water. Heat to boiling; reduce heat. Skim off foam. Cover and simmer 3 hours. Remove meat to warm platter. Discard bay leaves. Skim fat from broth. Purée broth in blender until smooth. Pour into saucepan. Stir in chocolate powder and heat through. Pour over oxtail pieces and serve immediately. If you wish to make additional sauce, add more red wine or beef broth. This dish demands a Torres Sangre de Toro, a heavy, full-bodied red wine from Catalonia.