Beat banana, Equal®, yogurt, margarine, egg, egg white, and vanilla at medium speed in large bowl until blended; beat at high speed 1 minute. Add combined flour, baking powder, baking soda, and salt, mixing just until ingredients are moistened. Stir in walnuts, if desired. Pour batter into one 8-1/2 x 4 inch greased and floured loaf pan, or three 5-5/8 x 3-1/4-inch loaf pans. Bake at 350°F until bread is golden and toothpick inserted in center comes out clean, 55 to 65 minutes for large loaf, 35 to 40 minutes for the small loaves. Cool 10 minutes in pans on a wire rack; remove from pan, and cool. Makes 1 loaf (8-1/2 x 4 inches) 14 slices or 3 loaves (5-5/8 x 3-1/4-inch)Recipe can be doubled to make 2 large or 6 small loaves.