Brown the butter. In a small saucepan over medium heat, melt the butter. Continue to stir occasionally until the mixture turns amber in color, then remove it from the heat and set it in the fridge for 10 minutes. You should be left with 90g of butter - if you have a little less (2-3g), just add in a bit of water until you reach 90g.
In a medium sized mixing bowl, mash up the banana with the cinnamon until only a few lumps remain.
Once the butter has chilled a little, add it to the mashed banana along with the brown sugar, granulated sugar, and vanilla. Mix until well combined.
Add in the flour, rolled oats baking powder, baking soda, and salt. Fold the mixture together using a rubber spatula just until only a few flour streaks remain.
Add in the chocolate chips, and fold the mixture together just until no flour streaks remain.
Cover the bowl with plastic wrap, and chill the dough in the fridge for at least 30 minutes, and up to 48 hours.**
Preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper or a silicone baking mat.
Use a 3 tbsp. cookie scoop to dish out heaping scoops of the cookies (or weigh out 80g cookies), leaving at least 2" between each cookie to allow room for spreading.
Top the cookies with more chocolate if desired. Bake for 12-14 minutes, or until the edges begin to turn golden brown.
Immediately after taking the cookies out of the oven, take a bowl that is slightly larger than the cookies and swirl it around over them to make them a little more uniform and circular.
Leave the cookies on the baking sheet for 5 minutes before transferring to a wire rack to cool. Serve and enjoy!