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 all-purpose flour, baking powder, baking soda, and salt until evenly incorporated.
03 - In a separate bowl, mash the ripe bananas thoroughly. Add the granulated sugar, vegetable oil (or melted butter), eggs, vanilla extract, and cooled espresso. Whisk until smooth and well combined.
04 - Pour the wet ingredients into the dry ingredients. Stir gently with a spatula just until combined—do not overmix. The batter should remain slightly lumpy.
05 - Fold in the chocolate chips and walnuts (if using) until evenly distributed throughout 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 a few moist crumbs.
08 - Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.