Ruth's Heavenly Chocolate Cake

1 pkg. chocolate cake mix
1/2 c. unsweetened cocoa
3 eggs
1 1/3 c. water
1 c. mayo

Heat oven to 350 degrees. Stir cake mix and cocoa; add remaining ingredients. Beat at medium speed with electric mixer until blended. Pour into greased and floured 13 x 9 or bundt pan. Bake 30 minutes-40 minutes or until toothpick inserted comes out clean.