In a large bowl, combine flour, sugar, cocoa, and baking soda. Add water, mayonnaise, and vanilla; beat at medium speed until thoroughly combined. Pour into a greased nine inch square pan or 11x7x2" pan. Bake at 350 for 30-35 minutes or until cake tests done. Cool completely before frosting. Frosting: Melt butter in a saucepan. Stir in brown sugar; cook and stir till bubbly. Remove from the heat and stir in milk. Gradually add powdered sugar; beat by hand until frosting is of spreading consistency. IMMEDIATELY frost cake, as it thickens quickly ... It is yummy!! ----