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, and cooled espresso. Whisk until smooth and fully incorporated.
04 - Pour the wet ingredients into the dry mixture. Fold gently with a spatula until just combined, taking care not to overmix as this can result in dense muffins.
05 - Fold in the chocolate chips and walnuts if using, distributing evenly throughout the batter.
06 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full.
07 - Bake for 18-22 minutes until a toothpick inserted into the center comes out clean or with just a few moist crumbs attached.
08 - Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.