Preheat an oven to 180C. Coat a 9-by-5-inch loaf pan with nonstick cooking spray. In a large bowl, combine the flour, baking powder, baking soda, and salt. Make a well in the center of the dry ingredients. In a medium bowl, mash the bananas until smooth. Add the egg whites, sugar, buttermilk, and vanilla and stir to mix well. Pour the banana mixture into the well in the dry ingredients and fold together until blended.