Preheat the oven to 350 degrees F (175 degrees C). Spray a 12-cup muffin tin with cooking spray.

Place ground beef, egg, milk, salt, and pepper in a large bowl; mix with your hands until well combined. Mix in cornmeal, brown sugar, onion, and ketchup until incorporated. Press mixture firmly into the prepared muffin cups, filling each cup 3/4 full.

Bake in the oven until no longer pink in the center, about 50 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).

Remove from the oven and allow to cool for 5 minutes before popping out of the muffin tin.