In a large bowl, cover the catfish with the buttermilk and refrigerate for 1 hour. Heat a large cast-iron skillet. One at a time, cook the tortillas over moderate heat until swarmed through, about 15 seconds per side. Stack the tortillas, wrap in foil and keep warm. On a large plate, combine the <em>masa harina</em>, cornmeal, cornstarch and salt. Remove the catfish strips from the buttermilk, shaking off any excess, and coat them with the cornmeal mixture.