Heat the remaining 2 tablespoons oil in a heavy-bottomed, 4-quart soup pot over medium heat. Add the onion, garlic, and bell pepper and saute about 5 minutes, until the vegetables become soft. Stir in the chili powder and cumin and cook for 1 minute. Add the chicken stock and ancho chile. Bring to a boil over medium-high heat, then cover, decrease the heat, and simmer about 15 minutes, until the ancho chile softens. Remove the chile from the soup and pull off and discard the stem. Cut the chile in half and discard the seeds, if desired. (Leaving the seeds in makes for a spicier soup.) Place the softened chile, the canned tomatoes with their juices, and the roasted tomatoes (including as much of the juices and the browned tomato bits as possible) in the bowl of a food processor fitted with a metal blade. Puree the chile-tomato mixture about 1 minute. Transfer the chile-tomato mixture to the soup and continue simmering, about 1 hour. Add salt nd pepper to taste, along with the cooked chicken. If using fresh corn, cut the kernels from the cobs, add the corn to the soup, and simmer for 5 minutes.