1Heat oven to 350°F. Spray 13x9-inch pan with cooking spray.
2In large bowl, beat cake mix, orange juice, pineapple juice, oil, 3/4 teaspoon orange peel and the egg whites with electric mixer on low speed 30 seconds, scraping bowl constantly. Beat on medium speed 2 minutes, scraping bowl occasionally. Pour batter into pan.
3Bake 25 to 30 minutes or until toothpick inserted in center comes out clean. Cool completely on cooling rack, about 1 hour.
4In large bowl, beat butter with electric mixer on medium speed until creamy. On low speed, beat in 2 cups of the powdered sugar. Add remaining 3 cups powdered sugar alternately with champagne, beating until frosting is smooth and spreadable. Frost cake. Garnish with additional orange peel. Store covered in refrigerator.