1. Preheat an oven to 350 degrees F. 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.
2. Pour the butter into the prepared loaf pan.
3. Bake until a toothpick inserted into the center comes out clean, 55-60 minutes. Cool for 10 minutes. To serve, cut into 8 slices. Store wrapped in plastic wrap in an airtight container in the freezer for up to 1 month.