01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease lightly.
02 - In a large bowl, whisk mashed bananas, eggs, vegetable oil, brown sugar, and vanilla extract until smooth.
03 - In a separate bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon if using.
04 - Gently fold dry ingredients into wet mixture until just combined; avoid overmixing.
05 - Fold in semi-sweet chocolate chips and nuts if desired.
06 - Divide batter evenly among muffin cups, filling each about three-quarters full.
07 - Sprinkle extra chocolate chips atop each muffin if preferred.
08 - Bake for 18 to 22 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
09 - Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.