Preheat oven to 350°F (175°C) and line two large baking sheets with parchment paper or silicone mats.

In a large mixing bowl, combine the mashed banana and melted butter. Whisk in the granulated sugar and brown sugar until the mixture is smooth.

Add the egg and vanilla extract to the banana mixture, whisking vigorously for 1 minute until the batter is glossy.

In a separate medium bowl, whisk together the flour, sifted cocoa powder, baking soda, and salt.

Gradually fold the dry ingredients into the wet ingredients using a spatula until no streaks of flour remain.

Fold in the semi-sweet chocolate chips.

Using a cookie scoop, drop approximately 1.5 tablespoons of dough per cookie onto the prepared sheets, spacing them 2 inches apart.

Bake for 10 to 12 minutes. The cookies are done when the edges are set but the centers remain slightly soft and fudgy.

Allow cookies to cool on the baking sheet for 10 minutes to structure before transferring to a wire rack for complete cooling.