In large bowl, whisk together all dry ingredients. Make a well in the center and pour in milk, egg yolk, vanilla, oil and mashed banana. Stir together until just combined. Whip egg white until stiff peaks form. Gently fold whites into batter. Stir in walnuts. Pour a heaping 1/3 cup into hot, greased waffle maker and cook to your preference. Serve with fresh banana slices, more walnuts and maple syrup.