Preheat oven to 425°F. Line a standard 12-cup muffin tin with paper liners and lightly spray with nonstick baking spray.
In a large bowl, use a fork to mash the bananas. Some chunks are okay.
Add the slightly cooled melted butter, granulated sugar, egg, and vanilla extract. Whisk together until fully combined.
In a separate medium bowl, whisk together flour, cocoa powder, baking soda, baking powder, and kosher salt.
Add the dry ingredients to the banana mixture. Use a rubber spatula to gently fold everything together until just combined. Do not overmix.
Fold in the chocolate chips.
Divide the batter evenly between the 12 muffin cups, filling each cup almost to the top (about ¾ full). Top with extra chocolate chips.
Bake at 425°F for 5 minutes.
Without opening the oven door, reduce the oven temperature to 350°F and continue baking for 10–12 minutes, or until the tops are set, spring back lightly when touched, and a toothpick inserted into the center comes out with a few moist crumbs, but no wet batter.
Let muffins cool in the tin for 5 minutes before transferring to a wire rack to cool completely.