Preheat oven to 350F/180C.Line a 8×8-inch or 9×9-inch square pan with aluminum foil, with extra sheet hanging out on both the sides and lightly grease it with non-stick cooking spray. Using aluminum foil will make it easy to take the baked bar out. If you don’t have foil, grease the pan generously with non-stick cooking spray.Place the bananas in a large bowl and mash it well with the back of a fork or a potato masher. Its not necessary that you mash it smoothly. Few chunks are okay.Stir in the oil, brown sugar and vanilla extract. Mix well with a wooden spoon.Now stir in the baking powder, baking soda and salt. Mix well.Next incorporate the flour and oats and mix. Some lumps are fine. You may have to add a tablespoon or two of water/milk to the batter if it appears to be thick.Spread the batter in the prepared pan. Bake it for 18-22 minutes or until a toothpick inserted in the center comes out clean.Remove the pan from the oven and place it on a wire rack to cool for about 10 minutes. Use the foil hanging out to lift the baked bar from the pan and place it on the wire rack to cool completely before slicing it into individual bars. If not doing the optional drizzle then you can cut it into slices once it is cooled completely.If you want to do the drizzle, melt white chocolate and the nut butter in 30 seconds increments in the microwave oven until it’s smooth. Place the melted chocolate in a zip-loc bag and cut the tip and drizzle it over the bar. Leave it for a while for the icing to harden and then cut it into individual slices.