Heat oven to 400. it is important the oven is at the right temperature for this recipe, so leave ample time to preheat. Grease and flour 9x13 pan. In a large bowl, combine sugar and flour. In a heavy pan bring butter, shortening cocoa and water to boil, then add buttermilk, vanill, cinnamon and eggs; mix well. the batter will be thin and may spatter. pour into pan and bake 400 for 25 minutes. For the frosting, bring the cocoa, butter and milk to boil. Pour into bowl and add sugar till the frosting reaches desired consistency. Stir in nuts if using. Spread over cooled cake.