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 350 degree F. 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.