01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper and set aside.
02 - In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar together using an electric mixer until the mixture is light and fluffy, about 2 to 3 minutes.
03 - Add the mashed bananas, egg, and vanilla extract to the creamed mixture. Mix on medium speed until all ingredients are well combined and smooth.
04 - In a separate bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt until evenly distributed.
05 - Gradually add the dry ingredient mixture to the wet ingredients, stirring gently with a spatula or wooden spoon until just combined. Avoid overmixing to keep the cookies tender.
06 - Gently fold in the rolled oats and chocolate chips or chopped walnuts if using, distributing them evenly throughout the dough.
07 - Drop tablespoon-sized mounds of dough onto the prepared baking sheets, spacing them about 2 inches apart to allow for spreading during baking.
08 - Bake in the preheated oven for 10 to 12 minutes, or until the edges are lightly golden and the centers appear just set. Do not overbake for the best chewy texture.
09 - Allow the cookies to rest on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely before serving or storing.