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 all-purpose flour, baking soda, salt, and cinnamon if using.
03 - In a large bowl, mash bananas, then add melted butter, sugar, eggs, and vanilla extract. Whisk until smooth.
04 - Gently fold dry ingredients into wet mixture until just incorporated, avoiding overmixing.
05 - Fold semisweet chocolate chips and chopped walnuts into the batter evenly.
06 - Divide batter evenly among 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 moist crumbs.
08 - Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.