Put sugar and flour in mixing bowl and mix dry. Combine in a small saucepan, margarine, shortening, cocoa, and water; bring to a boil. Pour hot mixture over dry ingredients and add buttermilk, baking soda, and eggs. Mix together and add cinnamon and vanilla. Mix well and pour into greased 9x13 pan. Bake at 400 degrees for 25 minutes.
For the frosting, use the same unwashed saucepan, and in it bring to a boil - cocoa, margarine and milk. Put powdered sugar into a separate bowl and pour hot mixture over it. Beat well. Add vanilla and pecans. Finally, (IMPORTANT) pour the frosting onto the cake while it is still hot.