Place the ancho and chipotle chiles in a saucepan, cover with water and set over medium-high heat until it comes to a boil. Simmer at medium heat for 10 minutes, until the chiles are completely rehydrated.
Place the rehydrated chiles into the jar of a blender, along with 1/2 cup of their simmering liquid, the garlic, cumin seeds, salt and olive oil. Puree until completely smooth. Scrape into a bowl and let it cool.
Place the chicken in a container and cover completely with the garlic and cumin marinade. You can marinate it covered in the refrigerator up to two days. But you may also roast it right away without marinating ahead of time.
If you marinated the chicken ahead of time, remove it from the refrigerator. Preheat oven to 450 degrees Fahrenheit.
Generously grease a baking pan with oil. Place the chicken on the pan skin side down and sprinkle with a little salt. Put it in the oven and roast for 15 minutes. Carefully remove it from the oven, reduce the temperature to 375 degrees, flip over the chicken pieces and spoon any of the marinade on the bottom of the pan over the chicken pieces. Pour the chicken broth onto the bottom of the pan and return to the oven. Roast for 45 to 50 more minutes, or until the chicken is cooked through and the juices run clear – and there are barely any juices when pierced with a knife.