Combine unsweetened cocoa and next 7 ingredients (through cinnamon) in a large bowl. Add beef; toss to coat. Remove beef from bowl. Lightly spoon flour into a dry measuring cup; level with a knife. Add flour to cocoa mixture; stir with a whisk. Return beef to bowl; toss to coat.
Heat olive oil in a Dutch oven over medium-high heat. Add beef mixture to pan; sauté 5 minutes, turning to brown on all sides. Remove beef from pan. Add onion and green bell pepper to pan; sauté 5 minutes or until tender. Add red wine and tomatoes; cook 3 minutes. Stir in beef broth, salt, and black pepper; return meat to pan. Cover, reduce heat, and simmer 1 hour and 10 minutes or until beef is tender, stirring occasionally. Garnish each serving with fresh oregano, if desired.