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 bowl, mash bananas until smooth. Add melted butter or oil, eggs, and vanilla extract. Whisk until fully incorporated.
04 - Pour wet ingredients into dry ingredients. Fold gently with a spatula until just combined—batter should remain slightly lumpy. Avoid overmixing to prevent dense muffins.
05 - Fold in chocolate chips and optional chopped nuts until evenly distributed throughout the batter.
06 - Divide batter evenly among the 12 muffin cups, filling each about three-quarters full.
07 - Bake for 18 to 22 minutes until golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
08 - Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.