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Pre-heat oven to 400 degrees. In a large bowl, mix sugar and flour.
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Heat butter, cocoa, water, and shortening to boiling; pour over the flour and sugar.
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Add buttermilk, baking soda, eggs, vanilla, and cinnamon to the bowl, mixing well. Pour into a 12x16 inch jelly roll pan or a high-sided cookie sheet. Bake for 20 minutes.
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When the cake comes out of the oven, get to work on the frosting. Heat butter, cocoa, and milk to boiling. Remove from heat. Sift in powdered sugar (if you don't have a sifter, add about a 1/2 cup at a time of the powdered sugar to the chocolate mixture) and whisk thoroughly, removing any lumps. Stir in vanilla and cinnamon.
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Pour frosting over slightly warm cake and allow it to sit for a few hours at room temperature before cutting into it.
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Serve with vanilla ice cream.