01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease lightly with cooking spray.
02 - In a large bowl, whisk together flour, oats, sugar, baking soda, baking powder, salt, and cinnamon until thoroughly blended.
03 - In a separate medium bowl, mash bananas until smooth. Whisk in melted butter or oil, eggs, and vanilla extract until uniform consistency.
04 - Pour wet mixture into dry ingredients. Fold gently with a spatula just until flour disappears—stop immediately when combined. Small lumps are acceptable.
05 - Gently fold in chocolate chips and optional chopped nuts until evenly distributed throughout batter.
06 - Divide batter evenly among 12 prepared muffin cups, filling each about three-quarters full.
07 - Bake for 18 to 22 minutes until golden brown and a toothpick inserted into center comes out clean or with moist crumbs.
08 - Let muffins rest in pan for 5 minutes. Transfer to wire rack and cool completely before storing.