Heat a large pot over medium heat. Add the ground beef, diced onion, and minced garlic. Cook until the beef is browned and the onion softens.
Stir in the chili powder, cumin, and salt. Cook for one minute to toast the spices.
Add the drained kidney beans and diced tomatoes. Bring the mixture to a simmer, then reduce heat and cook uncovered for 20 minutes, stirring occasionally.
While the chili simmers, preheat the oven to 400°F and grease a 12-cup muffin tin.
In a mixing bowl, whisk together the cornmeal, flour, baking powder, and sugar. In a separate bowl, combine the milk, egg, and melted butter. Pour the wet ingredients into the dry ingredients and stir just until combined.
Spoon the batter evenly into the muffin tin. Bake for 15 minutes until the tops are golden and a toothpick comes out clean.
Let the muffins cool for a few minutes before serving alongside bowls of the chili.