Preheat oven to 350 degrees F and prepare muffin pans with butter or paper liners
Sift flour, cocoa powder, baking powder, baking soda and salt into a mixing bowl
In another mixing bowl, whisk together the mashed bananas, light brown sugar, melted butter, sour cream, egg and egg yolk and vanilla extract until fully combined
Slowly add the dry ingredients to the banana mixture and whisk until all ingredients are combined and smooth
Fold in the chocolate chips to the chocolate banana muffin batter
Scoop batter into prepared muffin pan to fill approximately ¾ full
Bake for 22-24 minutes or until toothpick comes out clean
Allow the muffins to cool before serving