Preheat oven to 350˚F (180˚C). Lightly spray 9 x 5-inch (23 x 13-cm) loaf pan with no-stick cooking spray.
Combine flours, baking powder and baking soda in medium bowl; set aside.
Beat Becel® Buttery Taste margarine with brown sugar in large bowl with electric mixer on medium-high speed until light and fluffy, about 5 minutes. Beat in eggs, buttermilk and vanilla, scraping sides occasionally, until blended. Alternately beat in flour mixture and bananas until blended. Stir in walnuts. Spoon into prepared loaf pan.
Bake 60 minutes or until toothpick in center comes out clean and top is dark golden brown. Cool 10 minutes on wire rack; remove from pan and cool completely.