Preheat oven to 350F. Line two baking sheets with parchment paper. Set aside.
In a large bowl, whisk mashed banana, sugar, egg, yogurt, and vanilla until smooth. In a separate bowl, combine oats, flour, cocoa, baking soda, and salt. Beat flour mixture into liquid until a dough forms. Gently stir in chocolate chips.
Roll dough into 1-in balls and place on prepared baking sheets. Gently flatten. Bake cookies in batches at 350F 9-10 minutes (middles will still look soft---this is ok) Cool cookies on baking sheet 5 minutes, then transfer to a wire cooling rack. Enjoy warm!