Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the softened butter, brown sugar, and granulated sugar. Beat until light and fluffy, about 2-3 minutes.
Add the mashed banana, egg yolk, and vanilla extract. Mix until well combined.
In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add to the wet mixture, stirring just until incorporated.
Gently fold in the mini chocolate chips until evenly distributed.
Drop rounded balls of dough onto a baking sheet lined with parchment paper, leaving space between cookies.
Bake for 10-12 minutes or until edges are lightly golden. Centers may look underbaked but will firm up as they cool.
Allow cookies to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
