Preheat oven to 350 degrees F. Grease and flour two 9 inch round cake pans. Set aside.
In a medium bowl, combine flour, cocoa powder, baking soda, baking powder and salt, then set aside.
In a large bowl, combine sugar, eggs and vanilla. Beat with an electric mixer for approximately 3 minutes or until light and fluffy. Add mayonnaise to the mixture and beat on low for approximately 1 minute.
Add flour mixture to the sugar mixture, in three additions, alternating with adding the water, stirring well between each addition. Pour into prepared baking pans.
Bake for 30-35 minutes or until a toothpick inserted into the center of the cake comes out clean. Allow to cool in pans for 10 minutes, then remove from pans and cool on wire racks.
Cool completely prior to frosting.