Preheat the oven to 350 degrees F.
Grease a muffin tin with cooking spray and then dust each space with cocoa powder.
In a small bowl, mix the flour, cornstarch, baking soda, salt, cocoa powder and chocolate chips.
Stir well to combine.
On a plate, mash the bananas well.
In another bowl, whisk together the egg and Greek yogurt.
Stir in the melted coconut oil (make sure it is not too hot. slightly cooled is best after melting), vanilla extract, brown sugar, and the mashed bananas.
Combine wet and dry ingredients and mix until just combined. (avoid over-mixing which makes the muffins denser)
Separate the batter evenly among the 12 muffin tins.
Bake for 18-23 minutes or until a toothpick comes out clean when inserted into the center.
Allow to cool for about 10 minutes and remove from the muffin tin. (gently cut around the edges with a butter knife to help them out)