Preheat your oven to 350 degrees
In your stand mixer fitted with a paddle attachment, cream the butter with the granulated and brown sugar.
Add the banana, eggs and vanilla until combined.
Add the flour, corn starch, baking soda, cinnamon and salt and mix until just combined. Make sure not to overmix.
Stir in the oats and walnuts.
Cover your baking sheet with parchment paper and spoon tablespoon sizes of dough onto the cookie sheet. Lightly press to slightly flatten the cookie. Place each cookie about 2 inches apart.
Bake for 10-12 minutes. Allow the cookies to cool for a couple of minutes before removing from the cookie sheet.