Preheat the oven to 350 degrees F.
Butter an 8-inch square baking pan. Whisk the muffin mix and chili powder in a large bowl to blend. Mix in the milk and eggs. Stir in the tomatoes. Spread the batter in the prepared pan. Bake until the corn bread is firm to the touch and golden brown, about 30 minutes. Cool slightly. Cut into squares and serve hot.