Preheat oven to 350°.
Grease and lightly flour two 9 inch round cake pans*; set aside.
Beat cake mix, mayonnaise, water, eggs in large bowl with electric mixer on low speed. Beat on medium speed, scraping sides occasionally, 2 minutes.
Pour batter into prepared pans.
Bake 30 minutes or until a toothpick inserted in centers comes out clean. Cool on wire rack 10 minutes.
Remove cakes from pans and cool completely. Sprinkle, if desired, with confectioners sugar or fill and frost.
*Or, prepare cake mix as above in 13 x 9-inch baking pan and bake 35 minutes or until a toothpick inserted in center comes out clean.