Preheat your oven to 350°F (175°C).
In a large mixing bowl, cream together the softened unsalted butter, packed brown sugar, and granulated sugar until light and fluffy.
Add the mashed overripe banana and egg yolk to the creamed mixture, along with the vanilla extract, and mix until well combined.
In a separate bowl, whisk together the baking soda, salt, and all-purpose flour. Gradually add to the wet ingredients, stirring until just combined.
Fold in the mini chocolate chips until evenly distributed.
Portion out the dough onto a baking sheet lined with parchment paper, leaving space between each cookie.
Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden.
Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.
