Preheat oven to 375 degrees F. Line a baking sheet with a Silpat or parchment paper and set aside.

In a medium bowl, whisk together 1 1/4 cups all-purpose flour, 1/2 cup Dutch-process or unsweetened cocoa, 1/2 teaspoon baking soda, and 1/2 teaspoon sea salt. Set the dry mixture aside.

If your banana is not already mashed, mash 1 ripe banana until smooth and measure 1/2 cup; set aside.

In the bowl of a stand mixer (or a large mixing bowl if mixing by hand), combine 1/2 cup melted, slightly cooled unsalted butter, 1/2 cup granulated sugar, and 1/4 cup packed light brown sugar. Beat or whisk until the sugars are incorporated and the mixture is combined.

Add the mashed banana to the butter-sugar mixture and mix until combined. Add 1 large egg yolk and 2 teaspoons pure vanilla extract and mix until incorporated.

Add the dry ingredients to the wet mixture and mix on low speed (or fold with a spatula) until just combined and no large streaks of flour remain. Do not overmix.

Stir in 1 cup semi-sweet chocolate chips until evenly distributed.

Use a cookie scoop or spoon to portion about 2 tablespoons of dough per cookie. Roll each portion into a ball and place on the prepared baking sheet about 2 inches apart. Gently flatten the tops with the palm of your hand.

Bake for 10 minutes, or until the cookies are set at the edges but still soft in the center.

Remove the baking sheet from the oven and immediately sprinkle cookies with flaky sea salt. If desired, gently press a few chocolate chips from the dough onto the tops while the cookies are still warm.

Let the cookies cool on the baking sheet for about 5 minutes, then transfer them to a cooling rack to cool completely.