01 - Preheat oven to 350°F. Line a baking sheet 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 or whisk until the mixture is light and creamy.
03 - Add the mashed bananas, egg, and vanilla extract to the creamed mixture. Mix until well combined and smooth.
04 - In a separate bowl, whisk together the rolled oats, 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 until just combined. Be careful not to overmix.
06 - Fold in the chocolate chips or chopped walnuts if using, distributing them evenly throughout the dough.
07 - Drop tablespoon-sized portions of dough onto the prepared baking sheet, spacing them about 2 inches apart to allow for spreading.
08 - Bake for 10 to 12 minutes, or until the edges are golden brown while the centers remain soft and slightly under-set.
09 - Allow the cookies to rest on the baking sheet for 5 minutes, then transfer to a wire cooling rack to cool completely before serving.