Add the melted butter or vegetable oil, brown sugar, and mashed banana to a large mixing bowl. Stir until well combined.

Add the egg yolk and vanilla extract. Mix until the mixture looks smooth and evenly blended.

Add the all-purpose flour, baking soda, cinnamon, and salt. Stir gently just until no visible streaks of flour remain. Avoid overmixing.

Fold in the chocolate chips until evenly distributed throughout the dough.

Cover the bowl and allow the dough to rest for about 15 minutes. Meanwhile, preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

Scoop approximately 1-ounce portions of dough onto the prepared baking sheet, leaving about 2 inches between each cookie. The dough will be softer and wetter than traditional chocolate chip cookie dough.

Bake the Banana Bread Chocolate Chip Cookies for 10–11 minutes, until the edges are set and the cookies have puffed slightly. The centers should still look soft and a little underbaked.

Allow the cookies to cool on the baking sheet for several minutes before transferring them to a cooling rack. Let them cool further before serving.