Sift together flour, cocoa, baking soda and salt; set aside. Combine mayonnaise and sugar in a mixing bowl. Beat with electric mixer at medium speed until blended. Gradually beat in cold water and vanilla. Add dry ingredients to mayonnaise mixture, beating until well blended, about 2 minutes. Pour batter into greased 13x9x2 inch baking pan. Bake in preheated 180ºC . oven 40 minutes or until cake tests done. Cool in pan on rack. Frost with Fluffy chocolate Frosting. Cut in squares.
FLUFFY chocolate FROSTING: Combine brown sugar, light cream, and butter in a saucepan. Cook over medium heat, stirring constantly, until mixture comes to a boil. Boil 1 minute. Remove from heat. Stir in chocolate chips and vanilla. Beat with electric mixer at high speed until of a spreading consistency.