Preheat the oven to 350°F. Grease and flour two 9-inch cake pans.
In a large bowl, add the flour, sugar, cocoa powder, and baking soda. Take a whisk and whisk the dry ingredients to remove any lumps.
In a medium bowl, add the mayonnaise, vanilla, and water, and using a whisk, mix together until smooth.
Slowly add the wet mixture to the dry mixture and stir just until combined. Do not overmix.
Divide the batter between the two prepared pans.
Bake for 30 minutes or until toothpick or cake tester comes out clean.
Remove from oven and cool completely.
Spread icing over the first cake, smoothing out to the edges.
Place the second cake on top, with the top side up.
Ice with chocolate frosting.