01 - Preheat the oven to 350°F. Line a 12-cup muffin pan with paper liners or grease lightly with butter or cooking spray.
02 - In a large mixing bowl, whisk together the flour, baking powder, baking soda, and salt until well blended.
03 - In a separate bowl, mash the ripe bananas thoroughly with a fork. 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—stop as soon as the flour disappears. Do not overmix or muffins will become tough.
05 - Fold in the chocolate chips and chopped walnuts if using, distributing evenly throughout the batter.
06 - Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full. Use a scoop or spoon for consistent portioning.
07 - Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs. Tops should be golden and spring back when touched lightly.
08 - Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. Serve warm or at room temperature.