01 - Preheat the oven to 350°F. Line a 12-cup muffin pan with paper liners or grease lightly with cooking spray.
02 - In a large bowl, whisk together the flour, baking powder, baking soda, and salt until well blended.
03 - In a separate bowl, mash the bananas thoroughly. Add sugar, oil or melted butter, eggs, vanilla extract, and cooled espresso. Whisk until smooth and fully incorporated.
04 - Pour the wet mixture into the dry ingredients. Stir gently with a spatula just until combined—some lumps are fine. Do not overmix as this will result in dense muffins.
05 - Gently fold in the chocolate chips and walnuts if using, being careful not to overwork the batter.
06 - Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full.
07 - Bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs.
08 - Allow the muffins to rest in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.