Sift sugar and plain flour together in a large bowl. Mix together margarine, cocoa, and water in boiler and bring to boil. Pour this mixture over the flour and stir well. Add eggs, and soda dissolved in buttermilk. Mix well. Add cinnamon and vanilla. Mix well by hand and pour into a greased 9x13 inch pan. (Glass pan is best.) Bake at 400'F. for 20-25 minutes. Chocolate Fudge Icing: Melt together and boil cocoa, margarine milk. Add powdered sugar, vanilla, and pecans. Spread icing over cake in pan. Cut into squares.