Preheat the oven to 180 C / 350F and line a cookie sheet with parchment paper.
Mash the banana with a fork. Then blend all ingredients (except for the walnuts) in a bowl with an electric mixer until smooth.
Using a tablespoon, dollop the cookie batter onto the cookie sheet (1 tbsp per cookie).
Space the cookies out because the batter spreads. Top with the crushed walnuts if using.
Bake for 10-12 minutes or until very lightly browned on the sides. Remove from the oven and let cool COMPLETELY before handling.
