Heat oil over medium heat in a large saucepan. Add onion; cook for 3 minutes or until translucent. Add garlic; cook for 1 minute. Stir in cumin, cinnamon, salt, and pepper; cook for 1 minute.
Add canned diced tomatoes and chiles to saucepan; simmer for 10 minutes. Stir in tahini and chocolate until melted. Stir in chopped cilantro; remove from heat and cool slightly. Process in a blender or food processor for 30 seconds or until smooth. (Add some water if sauce is too thick.) Spoon sauce over chicken. Serve with avocado and tomato.
