In a medium bowl, stir together the banana and peanut butter until well blended. Stir in the dark brown soft sugar, caster sugar and vanilla until smooth. Combine the flour and baking powder; add to the peanut butter mixture and mix well. Cover and chill dough for at least 1 hour.
Preheat oven to 190 C / Gas 5. Line baking trays with baking parchment. Roll dough into walnut sized balls and place 5cm apart onto the prepared baking trays.
Bake for 10 to 12 minutes in the preheated oven. Allow cookies to cool on baking tray for 5 minutes before removing