Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease the cups.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt.
In a large bowl, combine the mashed bananas, granulated sugar, beaten egg, melted unsalted butter, and vanilla extract. Stir until well blended.
Add the dry ingredients to the banana mixture and stir just until combined. Do not overmix.
Fold in the semi-sweet chocolate chips evenly into the batter.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the pan for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.