Preheat oven to 350°F. In large bowl, combine cake mix and cocoa. Add eggs, water, mayonnaise and zest; beat with electric mixer on low speed 30 seconds, scraping bowl frequently. Beat on medium speed 2 minutes. Pour batter into prepared pan. Bake 18-20 minutes or until toothpick inserted in center comes out clean. Cool completely before decorating with icing.