Brown the beef and onion for 9 minutes until the beef reaches 160°F, drain, and stir in the garlic, chili powder and cumin.
Transfer to a buttered 6-quart crockpot and add the tomato sauce, black beans, corn and 1 teaspoon salt; cook covered on low for 5 hours.
Whisk the cornmeal, flour, baking powder and remaining salt, then stir in the egg, buttermilk and melted butter just until combined.
Switch the crockpot to high, spread the batter over the hot filling, and cover without opening for 60 to 75 minutes.
Check that the cornbread center reaches 200°F and a toothpick comes out without wet batter before scooping.