Lightly coat a 16 x 12 inch baking sheet with non-stick spray for baking.
Combine butter, shortening, cocoa, and water in a 2 quart sauce pan, and bring to a boil over medium heat. Stir frequently.
Whisk together flour, sugar, and salt.
Pour hot cocoa mixture over flour mixture and whisk until smooth.
Gently beat eggs, and add them, along with buttermilk, baking soda, cinnamon and vanilla to the cocoa mixture. Whisk just until incorporated—do NOT over-mix. The baking soda and buttermilk will begin to react to create a bubbly leavening, so work quickly.
Pour batter into prepared pan, and bake at 400 degrees for 20 minutes.
During the last 10 minutes of baking, prepare frosting.