In a large bowl, blend the softened butter and brown sugar until fully combined.
Add the mashed banana, then mix thoroughly to combine.
Incorporate the egg yolk and vanilla extract, mixing well to form a smooth batter.
Add the salt, baking soda, and cinnamon, stirring until evenly mixed.
Gradually fold in the flour until the dough has no dry streaks.
Gently fold in the chocolate chips and toasted walnuts for even distribution.
Cover the dough and refrigerate for one hour to firm up.
Preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper.
Portion the chilled dough with a 1.5 oz cookie scoop onto the baking sheets, spacing them 2 inches apart.
Optional: press extra chocolate chips onto each dough ball for added richness.
Bake for 10 to 12 minutes, until edges are golden but centers remain soft.
Cool cookies on baking sheets for 2 minutes before transferring to a wire rack to cool completely.