Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper and set aside.
In a medium-sized bowl, whisk together the flour, salt, baking powder, baking soda, cinnamon, and oats. Set this aside for a moment.
In the bowl of a stand mixer, cream the butter and sugars together. Beat for 2-3 minutes, or until it is light and fluffy.
Add the egg, vanilla extract, and mashed banana. Scrape down the sides of the bowl as necessary.
Little by little, add the dry ingredients to the wet, being careful not to over mix the dough.
Add the chocolate chips and walnuts and mix to combine.
Using a cookie scoop, portion the dough onto the prepared baking sheets. Top with some more chocolate chips, if you'd like. Bake for 9-12 minutes, or until the edges just begin to turn golden brown.
Allow the cookies to cool on the baking sheet for 5 minutes before removing them to a wire rack to finish cooling completely.