Preheat the oven to 350 degrees F.
Generously coat a muffin tin with cooking spray and then dust the cavities with cocoa powder. These muffins stick to muffin liners so I do not recommend using them.
Remove 1 tablespoon of the 1 cup of flour.
In a small bowl, toss together the remaining flour (after removing 1 tablespoon from the 1 cup), cornstarch, baking soda, salt, cocoa powder, and both types of chocolate chips. Stir.
Make sure your bananas are at their very ripest (black skins are best; see post to speed up the ripening process if not all the way ripe.) Remove the skins and mash the bananas extremely well.
In another bowl, briskly whisk together the egg and Greek yogurt. Stir in the melted coconut oil (slightly cooled, but measure in it's melted state),vanilla extract, brown sugar, and the extremely ripe mashed bananas.
Combine wet and dry ingredients and mix until JUST combined. Over-mixing results in denser muffins.
Separate the batter evenly among the 12 cavities or fill up 9-10 for extra large muffins.
Bake for 18-23 minutes or until a toothpick comes out clean when inserted into the center of a muffin.
Allow to cool and then remove from the muffin tin. (Use a butterknife to cut around the edges and help pull them out!)
Store in an airtight container in the fridge up to 3 days.
When ready to enjoy a muffin, microwave for 15-20 seconds. Spread with peanut butter if desired!