1. In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and brown sugar on low until pale and fluffy – about 10 minutes. Scrape down the sides of the bowl if needed.
2. Add the egg and vanilla and continue to beat on low an additional 5 minutes. Add the bananas and cinnamon and mix until combined. The batter should be pale and fluffy and almost look whipped.
3. Combine the flour, baking soda, baking powder and salt. Slowly add to the mixture until just incorporated. Careful not to over mix at this stage. Stir in the oats by hand and fold into the mixture.
4. Line a baking tray with parchment paper. Use a cookie scoop to scoop your batter out into about 1 tbsp portions ensuring to pack the scoop and scrape off any excess before turning out onto your baking sheet. It's important to keep the shape of the cookie scoop and the balls are nice and round and tall. Transfer the balls to the freezer and let chill 20 minutes.
5. Preheat the oven to 325 degrees F. Place a few balls on a second baking sheet, leaving about 2 inches between each ball. Bake for 14 minutes, removing from the oven half way at the 7 minute mark and gently tapping the sheet on the counter once or twice. Once the cookies have finished baking, let the cookies rest 1 minute in the tray and then transfer to a cooling rack to rest an additional 5 minutes.