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