Preheat oven to 400 degrees F. Spray a muffin tin with cooking spray.
Whisk together bananas, oil, egg, and sugar, making sure to break up any sugar lumps. Add baking mix and 1/2 c. cornflakes, and stir just until combined.
Scoop batter with a large disher or 1/3 c. measuring scoop into muffin tin. (I was only able to get 11 good sized muffins out of this recipe.)
Sprinkle 1 tbsp. of cornflakes over each muffin, and press cereal into batter slightly to help it stay on after baking. (You can crush the cornflakes before sprinkling them on top if desired, but we liked the large flakes.)
Bake for 15-18 minutes in preheated oven, or until golden brown and toothpick inserted into the center of the largest muffin comes out clean. Allow to cool slightly in pan before removing to serve warm.
The cereal topping on leftover muffins will lose its crunch. Reheat in a toaster oven or 350 degree oven for a few minutes, and cornflakes will crisp up perfectly again.