Start by mashing your bananas lightly if doing by hand, or just mix them with the melted butter, sugar, and egg using a mixer. The mixture will be moist and slightly loose, which is normal because of the bananas.
In a separate bowl, whisk together the flour, oats, baking soda, salt, and cinnamon. Slowly fold this into the wet mixture until everything is just combined. Overmixing can make the cookies tough.
Gently fold in your chocolate chips so that each bite has melty chocolate without breaking up the dough too much. I like to use a rubber spatula to get every last bit of dough mixed evenly.
Scoop cookie dough onto a parchment-lined tray and refrigerate for at least 2 hours. Chilling is essential because the dough is very soft; this prevents the cookies from spreading too much.
Preheat your oven to 350°F (175°C). Bake the chilled dough for 10-12 minutes, just until edges are set. Remember, the centers will still be soft, and they firm up as they cool.