In large bowl, beat cake mix, water, oil, buttermilk and egg whites with electric mixer on medium speed. Beat in melted white chocolate. Add about 20 drops blue food color to batter; mix well. Scrape down bowl; mix again until no white streaks remain. Pour batter into pans. Bake as directed on cake mix box for 2 (9-inch) rounds. Cool cakes completely before frosting.