01 - Preheat oven to 350°F (175°C). 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 the bananas. Add melted butter, granulated sugar, eggs, and vanilla extract; whisk until thoroughly combined.
04 - Fold the dry ingredient mixture into the wet ingredients gently until just combined, avoiding overmixing.
05 - Gently fold in the semisweet chocolate chips and chopped walnuts until evenly distributed.
06 - Divide batter evenly among prepared muffin cups, filling each approximately three-quarters full.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted into the center emerges clean or with a few moist crumbs.
08 - Allow muffins to cool in the tin for 5 minutes; then transfer to a wire rack to cool completely.