Heat the oven to 400°F and coat a 9-by-13-inch baking dish with cooking spray.
Stir the chicken, black beans, corn, enchilada sauce and salt together in the dish, then cover tightly with foil.
Bake for 25 minutes and verify that several center chicken pieces reach 165°F.
Push the chicken mixture level, then stir the corn muffin mix, egg and milk together in a large measuring cup just until no dry pockets remain.
Pour the batter evenly over the hot filling and return the dish to the oven uncovered.
Bake for 22 to 27 minutes, until the cornbread is deep gold and a pick inserted into the topping comes out clean, then rest for 8 minutes.