Brown the ground beef in a large pot over medium heat. Add the diced onion and minced garlic, then cook until the onion softens.
Stir in the chili powder, cumin, and salt. Add the black beans and diced tomatoes. Simmer uncovered for 20 minutes, stirring now and then.
While the chili cooks, heat the oven to 400 degrees. In a bowl, mix the cornmeal, flour, and baking powder. Whisk in the milk, egg, and melted butter until smooth.
Pour the batter into a greased 8-inch square pan. Bake for 15 minutes until the top turns golden. Let it cool for 5 minutes, then cut into sticks.
Ladle the chili into bowls and serve the cornbread sticks on the side for dipping.