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 on medium speed until light and fluffy, about 2 to 3 minutes.
03 - Add the mashed bananas, egg, and vanilla extract to the butter mixture. Continue mixing on medium speed until thoroughly combined and smooth.
04 - In a separate medium 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. Be careful not to overmix.
06 - Gently fold in the rolled oats and, if desired, the chocolate chips or chopped walnuts until evenly distributed throughout the dough.
07 - Drop rounded tablespoon-sized mounds of dough onto the prepared baking sheets, spacing them approximately 2 inches apart to allow for spreading.
08 - Bake for 10 to 12 minutes, or until the edges are lightly golden and the centers appear just set. Rotate the baking sheets halfway through for even baking.
09 - Let the cookies rest on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely before serving or storing.