Heat oil in a large saucepan over med. low heat. Add onion, garlic, oregano, cumin and cinnamon. Cover and cook until onion is almost tender, stirring occasionally, about 10 minutes. Mix in chili powder and flour, stir for 3 minutes. Gradually whisk in chicken broth. Increase heat to med. high. Boil until reduced, about 35 minutes, stirring occasionally. Remove from heat. Whisk in chocolate; season with salt and pepper, if desired.