Make topping: Preheat oven to 350°F. Combine all ingredients in small bowl. Mash with fork until blended and coarse crumbs form. Set aside.
Make cake: Spray 9-inch-diameter cake pan with 1 1/2-inch-high sides with vegetable oil spray. Sift flour, sugar, baking powder, baking soda and salt into large bowl. Whisk applesauce, buttermilk, oil, egg yolk, vanilla and lemon peel in medium bowl until well blended. Add to dry ingredients and stir just until moistened (batter will be thick). Beat egg whites in another large bowl until stiff peaks form. Whisk half of whites into batter. Fold in remaining whites.
Transfer batter to prepared pan. Sprinkle topping over batter. Bake cake until tester inserted into center comes out clean, about 30 minutes. Transfer to rack; cool in pan. Cut into 10 wedges and serve.
Per serving: calories, 196; total fat, 7 g; saturated fat, 3 g; cholesterol, 41 mg. Nutritional analysis provided by Bon Appétit