Preheat oven to 350°F (175°C).
Line a baking sheet with parchment paper or a silicone mat.
In a large bowl, cream together butter, brown sugar, and granulated sugar until smooth.
Add mashed bananas, egg, and vanilla extract, mixing well.
In a separate bowl, whisk together flour, baking soda, salt, and cinnamon.
Gradually add dry ingredients to the wet mixture, stirring until just combined.
Fold in chocolate chips or walnuts (if using).
Drop 1.5 tbsp-sized dough balls onto the baking sheet, spacing them 2 inches apart.
Bake for 12-15 minutes or until edges are lightly golden.
Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack.
Enjoy warm or store in an airtight container for up to 3 days.
