Combine sugar, flour, cinnamon, salt and soda in large bowl and set aside. In saucepan bring butter, cocoa, and water to boil and pour over flour-sugar mix. Stir in eggs, buttermilk, and vanilla. Pour into large thin cookie sheet. Bake at 350 for 20-30 min. Make sure you start checking it at 20 minutes.
In sauce pan bring marg, cocoa, and milk to a boil and remove from heat. Add powdered sugar and beat until creamy. Add vanilla. Ice while hot.
For smoother frosting sift powder sugar and whisk rapidly while pouring into cooked ingredients.