Preheat oven to 375℉/190℃. Line a baking sheet with parchment paper and set aside.
In a large bowl, whisk together the melted butter and sugars until combined. Add the mashed banana, egg yolk and vanilla and continue whisking. It's ok if there are chunks, that's due to the banana!
1/2 cup butter, 1/4 cup white sugar, 1/3 cup light brown sugar, 1 banana, 1 egg yolk, 1 tbsp vanilla
Add in flour, cocoa powder, baking soda, and salt. Using a spatula, gently fold until no flour streaks appear.
1/2 cup unsweetened cocoa powder, 1 1/4 cups all-purpose flour, 3/4 tsps baking soda, 1/4 tsp salt
Toss in the chocolate chips and continue mixing.
1/2 cup semi-sweet chocolate chips
Using a large cookie scoop (2 or 3 tbsp), scoop 6 cookies onto the prepared baking mat, about 2 inches apart.
Bake for 13-15 minutes. The cookies will be ready when the edges are set but the middle looks soft.
Add sea salt on top along with extra chocolate chips (optional). Allow to sit on the tray for 10 minutes after baking before transferring to a wire rack.
Enjoy!