In a bowl, mix brown sugar and butter.
In another bowl mash the bananas with a fork. You can also employ a processor for a smooth puree.
Add the banana puree to the butter-sugar mixture. Whisk till well combined.
Into this add the oats, cornflour, baking powder, and chocolate chips/buttons. Mix with a spatula till well combined.
Cover the bowl with a cling wrap and chill for 30 minutes.
Preheat the oven to 180℃/350℉. Line a baking tray with parchment/butter paper.
Scoop the cookie dough with a scooper (2-inch diameter)/spoon (2 tablespoons each) and place them on the baking tray with a little distance between them. Press them down a little and place some chocolate chips/buttons on top.
Bake at 180℃/350℉ for 18-20 minutes or until the surface/edges get slightly golden.
Let them cool completely on the tray itself before serving or storing.