01 - Preheat oven to 350°F. Line a 12-cup muffin pan with paper liners or grease lightly.
02 - Whisk together flour, baking powder, baking soda, and salt in a large bowl.
03 - Mash bananas in a separate bowl. Add sugar, oil or melted butter, eggs, vanilla, and espresso. Mix until well combined.
04 - Pour wet ingredients into dry ingredients. Stir gently until just combined—do not overmix.
05 - Fold in chocolate chips and walnuts if using.
06 - Divide batter evenly among muffin cups, filling each about 3/4 full.
07 - Bake for 18-22 minutes until a toothpick inserted into center comes out clean or with few moist crumbs.
08 - Let muffins cool in pan for 5 minutes, then transfer to wire rack to cool completely.