Preheat the oven to 350° F/175° C. Pat the chicken breasts dry and sprinkle on chili powder until well coated. Place chicken in the prepared 9×13 pan and bake in the preheated oven for about 20-30 minutes. Remove from oven and allow to cool for a few minutes, then shred with a fork.
Change the oven to broil. Spread the tomatillos out on the lined baking sheet stem side down. If you use frozen tomatillos, slice each in half and allow some of the juice to drain off, then place face down on the lined baking sheet. Place under broiler until charred, about 8 to 10 minutes. If using whole fresh tomatillos, some will crack and release juice, and that’s ok. Add half of the roasted tomatillos and juice to the blender jar.
On a dry cast iron skillet, toast the onion, anaheim and jalapeno chilies, and garlic, turning until charred on all sides. Remove the stems from the anaheim and jalapenos, then cut into chunks. (Note: Remove the seeds of the chiles if you prefer mild heat.) Add half the tomatillos, onions, cilantro, chilies, and garlic to the blender jar. Blend until there are no chunks left, then pour it into the 6-quart pot. Repeat with the remaining tomatillos, onions, cilantro chilies, and garlic.
Add the chicken stock to the pot and stir, then add the shredded chicken along with the cumin, chipotle powder, pepper, and salt. Cook over medium heat about 20 to 30 minutes. Taste it after 10 minutes and decide if you want to add the optional hot pepper sauce. Serve hot with the garnishes.