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