Heat the oven to 375°F and butter a 3-quart baking dish with 1 teaspoon of the melted butter.
Brown the beef and onion in a large skillet over medium-high heat for 8 to 10 minutes, until the beef reaches 160°F, then drain the fat.
Stir in the beans, tomatoes, tomato sauce, corn, chili powder and 1/2 teaspoon salt and simmer for 8 minutes, until thick but still spoonable.
Spread the beef filling in the prepared dish.
Whisk the cornmeal, flour, baking powder and remaining salt in a bowl, then stir in the milk, egg and remaining melted butter just until combined.
Drop the batter in large spoonfuls over the filling and spread it carefully to the edges.
Bake for 35 to 40 minutes, until the cornbread is golden, its center reaches 160°F and a pick inserted only into the topping comes out clean.
Rest for 10 minutes before cutting through the crust.
