01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.
03 - Add the mashed bananas, egg, and vanilla extract to the creamed mixture. Beat until well combined.
04 - In a separate bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt.
05 - Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Do not overmix.
06 - Fold in the rolled oats and chocolate chips or chopped walnuts if using, distributing evenly throughout the dough.
07 - Drop tablespoon-sized mounds of dough onto the prepared baking sheets, spacing them about 2 inches apart.
08 - Bake for 10 to 12 minutes, or until the edges are golden and the centers appear just set.
09 - Allow the cookies to rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.