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 mixing bowl, whisk together the flour, baking powder, baking soda, and salt until well combined.
03 - In a separate bowl, mash the ripe bananas thoroughly. Add the sugar, oil or melted butter, eggs, vanilla extract, and cooled espresso. Whisk until smooth and fully incorporated.
04 - Pour the wet ingredients into the dry ingredients. Using a spatula, fold gently until just combined. The batter should remain slightly lumpy—do not overmix or the muffins will become tough.
05 - Gently fold in the chocolate chips and walnuts if using, distributing them evenly throughout the batter.
06 - Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full. Use an ice cream scoop for consistent portioning.
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. The tops should be golden and spring back lightly when touched.
08 - Let the muffins cool in the pan for 5 minutes, then carefully transfer them to a wire rack to cool completely before serving.