Preheat oven to 350°. Mix together first 3 ingredients, set aside. In small pan, melt margarine, add cocoa and water; bring to a boil. Remove from heat, add eggs, buttermilk and vanilla; mix. Add to dry ingredients and mix well. Pour onto a greased and floured cookie sheet. Bake15 to 20 minutes.
Melt margarine, add cocoa; boil. Cool slightly. Add rest of ingredients; mix. Spread while cake and icing are warm.