Preheat and Prep: Heat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Mix the Wet Ingredients: Cream the softened butter, brown sugar, and granulated sugar until fluffy. Stir in the mashed banana, egg, and vanilla extract until smooth.
Prepare the Dry Ingredients: In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt.
Combine Everything: Gradually mix the dry ingredients into the wet mixture until just combined. Avoid overmixing to keep the cookies soft.
Add the Mix-Ins: Fold in the chocolate chips and walnuts (if using).
Scoop and Bake: Drop spoonfuls of dough onto the baking sheet, spacing them 2 inches apart. Bake for 10-12 minutes until the edges are golden brown.
Cool: Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack.