01 - Preheat your oven to 350°F (175°C). 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 the bananas until smooth. Whisk in melted butter or oil, eggs, and vanilla extract until fully incorporated.
04 - Pour the wet ingredients into the dry ingredients. Using a spatula, fold gently until just combined—do not overmix or muffins will become tough.
05 - Fold in the chocolate chips and chopped nuts if using. Reserve a few chocolate chips or oats to sprinkle on tops if desired.
06 - Divide batter evenly among the 12 muffin cups, filling each about three-quarters full.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin 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.