In a bowl, whisk together the flour, cocoa powder, baking soda, and salt until well combined.
Beat the butter with the white sugar, light brown sugar, and vanilla extract until a paste-like consistency.
Add the mashed banana and mix to combine. Then add the egg and beat again.
Add the dry ingredients and mix until just combined.
Fold in the chocolate chips, cover the bowl with plastic wrap, and refrigerate for 20–30 minutes or until the dough is firm enough to shape into balls.
Preheat the oven to 350F (180C) and line two baking sheets with parchment paper.
Scoop 16 dough balls (I use a 3 tablespoon cookie scoop) onto the prepared baking sheets, spacing them about 2 inches apart.
Bake in two batches for 11 minutes or until the cookies are puffy, have finished spreading, and the tops no longer look wet.
Remove from the oven and allow to cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely.