01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or lightly grease with nonstick spray.
02 - In a medium bowl, whisk together flour, baking soda, salt, and cinnamon if using.
03 - In a large bowl, mash the bananas then add melted butter, sugar, eggs, and vanilla. Whisk until fully blended.
04 - Gently fold the dry ingredients into the wet mixture until just combined, taking care not to overmix.
05 - Fold in the semisweet chocolate chips and chopped walnuts evenly throughout the batter.
06 - Divide batter evenly among the prepared muffin cups, filling each about three-quarters full.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
08 - Allow muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.