1. Sprinkle chicken with black pepper. Heat 2 tablespoons oil over high heat in a deep 4-quart pot. Add chicken and cook until just cooked through, stirring a few times. Set chicken aside in a covered bowl to stay warm.
2. Reduce heat to medium-high and add remaining oil to pot. Add chili powder, cumin, cinnamon, and cayenne, if using. Mix well, adding a little more oil if needed until spice mix is wet with oil. Heat spices until they're blackened and smoking, 4 to 6 minutes.
3. Reduce heat to low and add chocolate. Stir into spice mixture with a heatproof rubber spatula.
4. When chocolate is fully melted, add tomatoes and garlic. Bring to a boil and simmer over medium heat for 10 minutes, stirring occasionally.
5. Stir cooked chicken and green peppers into sauce and simmer 5 more minutes. Serve over hot rice.