In a mixing bowl combine buttermilk, eggs, vanilla, and baking soda. Whisk until smooth and set aside.
In another mixing bowl combine then sift flour, sugar, cinnamon, and salt. Set aside.
Bring to boil in saucepan unsalted butter, vegetable shortening, cocoa, and water.
Pour hot mixture over flour mixture and stir vigorously with a wooden spoon. Add buttermilk mixture and stir to thoroughly incorporate.
Pour batter in buttered and floured half-sheet-cake pan (about 15 x 10) and bake at 400 degrees for 20 minutes or until edges of cake pull away from the pan and the cake springs back when you touch it.