In a large mixing bowl, blend together the melted butter or vegetable oil, light brown sugar, and mashed banana until smooth and creamy, about 2-3 minutes of vigorous whisking.

Incorporate the egg yolk and vanilla extract into the mixture, stirring until combined and glossy, scraping the bowl to ensure all ingredients are mixed, about 1-2 minutes.

Sift in the all-purpose flour, baking soda, ground cinnamon, and salt, gently folding the dry ingredients into the wet mixture until no dry flour remains.

Fold in the chocolate chips gently, then let the dough rest for 15 minutes to help hydrate the flour.

Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

Scoop tablespoon-sized balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.

Bake the cookies for 10-11 minutes until the edges are set and the tops look slightly underbaked.

Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack.