Preheat oven to 180 degrees C. Grease and flour two 23cm round cake tins.
Sift flour, salt, bicarb soda and cocoa together.
Whisk 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 for 20 to 25 minutes. Ice with your favourite icing.