Start by browning half the butter in an sauce pot. This should take 2-3 minutes. Take off the heat and let it cool.
In a mixing bowl beat the other half of the butter with brown sugar, castor sugar, cinnamon, vanilla until pale and creamy, around 2 minutes.
Add the cooled brown butter and beat again until combined. Add the mashed banana and beat for 2 minutes.
Add the flour, baking powder salt, dark chocolate and corn flour, stir to combine until everything is smooth, place the dough in the fridge for 15 minutes and preheat the oven to 190℃.
Line a baking tray with baking paper, portion out the cookie dough with an ice cream scoop and keep plenty of space between them as they will spread as they bake.
Bake for 13-15 minutes - half way through baking baking the pan on the kitchen counter to flatten and deflate the cookies.
Once they come out of the oven, if you want those perfectly round edges, run a cookie cutter/large circular glass around them and allow the cookies to cool on the tray for 5 minutes before transferring with a metal spatula to a wire rack.