Preheat the oven to 350F and prepare a large baking sheet by covering it with parchment paper.
In a large bowl, combine the softened butter, brown sugar, and granulated sugar. Stir with a wooden spoon until the mixture is creamy.
Add the egg and vanilla extract and stir until combined.
Add in your bananas and mash with the back of your spoon. Stir until the mix is combined and there are no large pieces of banana. The batter might look a little curdled but that is totally fine, it will fix itself when you add the dry ingredients.
Stir in the dry ingredients (flour, baking soda, salt, cinnamon) until combined.
Stir in the oats and any mix ins. The dough will seem very wet, like a cake batter. That is what you want!
Scoop 2.5 tablespoon amounts of cookie dough onto the baking sheet. This is a LOT easier if you have a cookie scoop. These cookies will spread a bit so space them a few inches apart.
If you want more of a muffin top type of feel, you can sprinkle the tops with coarse sugar before baking.
Bake for 10-12 minutes, or until the edges and bottoms are golden brown and set. The center should still be soft.
Transfer to a cooling rack and allow to cool completely.