1. Preheat oven to 350°. Grease and lightly flour two 9-inch round cake pans*; set aside.
In medium bowl, combine flour, cocoa, baking soda and baking powder; set aside.
In large bowl, with electric mixer at high speed, beat eggs, sugar and vanilla for 3 minutes or until light and fluffy. Beat in Hellmann's® or Best Foods® Real Mayonnaise at low speed until blended. Alternately beat in flour mixture with water, beginning and ending with flour mixture. Pour into prepared pans.
Bake 30 minutes or until toothpick inserted in centers comes out clean. On wire racks, cool 10 minutes; remove from pans and cool completely. Frost, if desired, or sprinkle with confectioners sugar.
*Or, prepare cake mix as above in 13 x 9-inch baking pan and bake 40 minutes or until toothpick inserted in center comes out clean.