Preheat oven to 350°. Grease 9 x 5-inch baking pan with no-stick cooking spray; set aside.

Whisk egg, brown sugar, bananas, mayonnaise and vanilla in large bowl with wire whisk until blended; set aside.

Combine flours, baking powder and salt in medium bowl; stir in ¾ cup walnuts. Fold into banana mixture just until combined. Turn into prepared pan; smooth top. Sprinkle with remaining ¼ cup walnuts.

Bake 1 hour or until toothpick inserted in center comes out clean. Cool on wire rack 15 minutes, then remove from pan and cool completely.