Heat a large cast-iron skillet over medium-high heat. Add the guajillo and ancho chiles and toast for 1 to 2 minutes, turning frequently. Discard and replace any burnt chiles.
Meanwhile, in a small pot, bring 1 cup (240 ml) of the beef stock to a boil.
Transfer the toasted chiles to a heatproof bowl, add the hot beef broth, and let the chiles soak unt