Position rack in center of oven and preheat to 350° F. Spray 10-inch-diameter springform pan with nonstick vegetable oil spray. Lightly dust pan with flour, tapping out excess. Finely chop walnuts and orange peel in processor using on/off turns. Combine flour, cocoa, baking powder and salt in large bowl. Stir in walnut mixture. Using electric mixer, beat eggs and egg whites in another large bowl until frothy. Gradually add 1 cup sugar and beat until mixture is pale yellow and thick and slowly dissolving ribbon forms when beaters are lifted, about 5 minutes. Slowly beat in olive oil on low speed. Beat in orange juice and vanilla. Gently fold egg mixture into flour mixture until thoroughly blended. Pour batter into prepared pan. Bake until edges of cake begin to pull away from sides of pan and center is firm to touch, about 35 minutes. Cool cake completely in pan on rack. Loosen sides of cake from pan with small knife. Remove rim of pan and transfer cake to serving platter. Sift powdered sugar over top of cake. Serve in small wedges. Serves 16. Per serving: Calories 230, fat 16g, sodium 51mg, cholesterol 27 mg PC 121