In a large mixing bowl, gently mash one small overripe banana until smooth, then whisk in 1/4 cup of melted butter or vegetable oil, along with 1/2 cup of light brown sugar. Mix thoroughly until well combined and glossy.

Next, add in the egg yolk and 1 teaspoon of pure vanilla extract to the banana mixture. Blend until fully incorporated.

Gradually stir in 1 cup of all-purpose flour, 1/2 teaspoon of baking soda, 1/4 teaspoon of salt, and 1/2 teaspoon of ground cinnamon. Mix until no dry flour streaks remain.

Gently fold in 1/2 cup of chocolate chips using a spatula.

Cover the bowl with a clean kitchen towel and let the cookie dough rest for approximately 15 minutes. Meanwhile, preheat your oven to 350°F (175°C).

Line a baking sheet with parchment paper. Use a cookie scoop or two tablespoons to drop rounded balls of dough onto the lined baking sheet, spacing them about 2 inches apart.

Bake for 10 to 11 minutes, or until the edges are golden and the centers look slightly underbaked.

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