Place chilies in large saucepan; add water to cover.
Bring to a boil, remove from heat and let stand 1 hour.
Cover chilies with a plate to submerge them.
Season chicken on both sides with salt and pepper.
Heat a large saucepan or dutch oven over medium heat; add olive oil to coat bottom of pan.
Add chicken, and lightly brown on all sides; then add 4 cups water and 1/4 onion.
Bring to a boil, reduce heat and simmer for 1 hour.
Grind sesame seeds, cloves, peppercorns, cinnamon, oregano, and thyme until pulverized.
Set aside.
Drain soaked ancho chilies and remove stems and seeds.
Puree chilies in blender, adding a little water if needed.
Pass through a sieve to strain out the peels.
Without washing blender, puree onion and tomato.
Remove chicken and strain broth into a bowl or pot.
Add pureed chilies and tomato-onion mixture to pan; cook over medium heat for about 7 minutes, stirring occasionally.
Add ground spice mixture, 1 1/2 cups of reserved broth, chocolate.
sugar, and salt to taste.
Add the meat; cover and simmer for 30 minutes.
Add more broth as desired for consistency.
Serve with soft tortillas and steamed rice, if you desire.