In a large bowl, combine melted, cooled butter with brown sugar, stirring until the mixture is smooth and creamy, about 2-3 minutes.

Add the mashed ripe banana to the butter-sugar mixture, stirring until thoroughly blended, roughly 1-2 minutes.

Stir in the egg yolk and vanilla extract, mixing until fully combined about another minute.

Now add salt, baking soda, and cinnamon if desired, stirring well to combine for about 30 seconds.

Gradually fold in the all-purpose flour using a spatula until no dry streaks remain, taking care not to overmix.

Gently blend in the chocolate chips and walnuts (if using) until evenly distributed throughout the dough, about 1 minute.

Cover the dough with plastic wrap and refrigerate for at least 1 hour.

Once the dough is chilled, line your baking sheets with parchment paper.

Scoop tablespoon-sized portions of dough onto the prepared sheets, keeping them spaced apart.

Bake the cookies in your preheated oven for 10-12 minutes or until the edges turn golden while the centers remain slightly soft.

After baking, allow the cookies to cool on the baking sheets for about 2 minutes. Then, carefully transfer them to a wire rack to cool completely.