Preheat oven to 350 degrees F (175 degrees C). Grease and flour two 8 or 9 inch round cake pans.
Sift all-purpose flour, salt, baking soda, and cocoa together.
Whip mayonnaise, white sugar, cold water, and vanilla together until mixed thoroughly. Add flour mixture slowly and beat at medium speed of an electric mixer for 2 minutes. Pour batter into prepared pans.
Bake at 350 degrees F (175 degrees C) for 20 to 25 minutes. Frost with your favorite frosting.