Preheat the oven to 350 degrees F (175 degrees C). Line 18 muffin cups with paper liners.
Pour melted butter into the bowl of an electric mixer and add brown sugar, eggs, and bananas. Mix until combined. Mix in all-purpose flour, oat flour, cocoa, espresso, cinnamon, salt, and baking soda until combined.
Spoon batter evenly into the prepared muffin cups.
Bake in the preheated oven until tops spring back when lightly pressed, about 20 minutes.