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

In a medium bowl, whisk together the flour, cocoa, baking soda, and salt. Set aside.

In the bowl of a stand mixer, beat together the melted butter and sugars until combined. Add the mashed banana and mix until combined. Add the egg yolk and vanilla and mix again.

Add the dry ingredients and mix on low until just combined. Don’t over mix. Stir in the chocolate chips.

Use a cookie scoop or spoon to scoop the dough into balls, about 2 tablespoons of dough per cookie. Roll into a ball and place on the prepared baking sheet, about 2 inches apart. Gently flatten the tops of the cookies with the palm of your hand. Bake the cookies for 10 minutes or until set but still soft in the center. Remove from the oven and sprinkle cookies with flaky sea salt. You can also gently press a few chocolate chips onto the tops of the cookies to make them extra pretty while the cookies are still warm.

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