If using chiles, trim the stems and remove seeds. Place in a small saucepan and add water to barely cover. Bring to a boil, remove from the heat, cover and let stand for 15 minutes. Transfer the chiles and their soaking water to a blender or a food processor fitted with metal blade. Purée until smooth. Set aside.

Brown half of the meat in a large skillet in the vegetable oil over high heat for 6 to 8 minutes. Transfer the meat and juices to a heavy pot and add the puréed chiles or chili powder, if using. Place over low heat and bring to a simmer. Meanwhile, brown the remaining beef in the same manner, then transfer it and the juices to the pot. Add enough stock or water to just cover the meat. Bring to a boil, reduce the heat to low and simmer, uncovered, for 30 minutes.

Add the garlic, onion, cumin, oregano, salt to taste, paprika and cilantro and continue to simmer, uncovered, stirring occasionally, until the meat is very tender, another 30 minutes. Add a little liquid if the mixture begins to stick or looks too dry.