Preheat oven to 350℉. Line sheet trays with parchment paper or a silicone baking mat.
Cream together the butter, brown sugar, and sugar in a large bowl with an electric hand mixer. Add the mashed bananas, eggs, and vanilla. Mix well and scrape bowl.
In a separate bowl stir together the flour, baking soda, cinnamon, and salt. Add to wet ingredients and stir until just incorporated. Fold in the oats and chocolate chips. The dough will be thick and sticky.
Use a medium cookie scoop (#40 scoop; about 1.5 Tablespoons) to spoon the cookie dough into a ball. (A trigger scoop is recommended due to the stickiness of the dough.) Place the cookies several inches apart onto a prepared baking sheet. Top with additional chocolate chips, if desired.
Bake at 350℉ for 10-12 minutes. Edges and bottoms will be lightly browned, but the tops may appear slightly dark/wet still. Do not over-bake. Leave cookies on tray for 5 minutes to finish setting up before transferring to a wire rack to cool completely.
Notes
Store in an airtight container at room temperature for up to 3 days or in the fridge for up to 1 week.