Put flour, sugar, salt and cinnamon in large bowl of mixer. In a saucepan, put margarine and shortening, water and cocoa. Bring to a boil and pour over flour mixture. In another bowl, put eggs, soda, buttermilk and vanilla. Add to preceding mixture and beat well. Pour into prepared sheet pan, 15 1/2 x 10 1/2 inches, or 13×9 inches. Bake large pan at 350 for 20 minutes; small pan at 350 for 35-to 40 minutes. Start icing 5 minutes before cake is done.
Mix margarine, cocoa and milk in saucepan. Heat over low heat, but do not boil. Remove from heat. Add powdered sugar, vanilla and pecans. Mix well and put on cake as soon as removed from the oven. Cut cake into squares to serve. Makes 24 servings.