In a pan over medium heat, melt the butter. Add the oats and cook, stirring occasionally, until the butter turns dark brown.
Pour the browned butter into a bowl, add the mashed banana, and stir together. Let cool until slightly warm to the touch.
Add the sugars and whisk until glossy.
Add the flour, baking soda, baking powder, cinnamon, and salt, and mix in with a rubber spatula.
Add the chocolate, making sure the dough is no longer warm so it doesn't melt in.
Using a cookie scoop, scoop the dough onto a baking sheet — 12 smaller (45 g) or 9 bigger (60 g) cookies. Chill uncovered for 1 hour (cover if chilling overnight).
Preheat the oven to 350°F.
Line a baking sheet with parchment and arrange 6 dough balls per batch. Bake 12-14 minutes depending on size.
Remove from the oven and slam the baking sheet against the counter 1-2 times to de-puff the cookies. Let cool on the pan until firm enough to move, then top with flaky salt.
