Preheat oven to 350°F (180°C). Grease and lightly flour two 9-inch/ 23 cm/ 1.5 L, round cake pans*; set aside.
Combine flour, cocoa, baking soda and baking powder in medium bowl; set aside.
Beat eggs, sugar and vanilla in large bowl with electric mixer at high speed until light and fluffy, about 3 minutes. Beat in Hellmann's® 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. Cool 10 minutes on wire racks; remove from pans and cool completely.
Frost with your favorite frosting or generously sprinkle with confectioners sugar.