Preheat oven 180ºC . Coat 23 cm square baking pan with vegetable cooking spray. Combine flour, cocoa, milk powder, baking soda and salt in bowl. Puree banana, sugar, egg whites, buttermilk and vanilla in food processor until smooth. Add dry ingredients and pulse just until blended. Pour into prepared pan. Bake 25 minutes or until toothpick comes out clean. Cut into 2 in. squares.