Spray a 12-cup muffin tin with cooking spray, or line with muffin cups. Preheat the oven to 350 degrees.

In a medium mixing bowl, mash the bananas with a potato masher or fork until no chunks remain. Mix with the sugars, melted butter, milk, and egg.

In a separate medium bowl, mix together the all-purpose flour, oat flour, cocoa powder, baking soda and salt. Add the wet ingredients, and mix together with a wooden spoon or spatula. The batter will be the consistency of a very thick cake batter. If it is too thick, add 1 or 2 tablespoons more milk, or if too thin, add 1 or 2 tablespoons more flour. Mix only enough to incorporate the dry ingredients, and do not overmix. Fold in the chocolate chips.

Fill the muffin cups about ¾ full. Bake for 15-18 minutes, or until cooked through and beginning to brown. Muffins are done if they spring back when you press lightly on the top with your finger, or if a wooden skewer inserted in the center comes out clean.

Allow the muffins to cool for 10-15 minutes, then remove muffins from the pan and let them finish cooling on a cooling rack.

Muffins taste best if eaten within 1 or 2 days, or keep in the freezer for longer storage.