Preheat oven to 400° F. Grease a half sheet sized baking pan; set aside. In a small saucepan, combine margarine, oil, water and unsweetened cocoa. Cook over high heat, stirring constantly, until boiling. Remove from heat and set aside to cool for 5 minutes. Pour into large mixing bowl. Add sugar, flour and baking soda and mix well. Add eggs one at a time, mixing well after each addition. Add salt, cinnamon, vanilla and buttermilk, beating until just combined. Transfer batter to prepared pan. Bake for 15-17 minutes or until a toothpick inserted near the center comes out clean. Cool cake for 30 minutes, then frost. Frosting: In a medium saucepan, combine butter, milk, and cocoa powder. Cook over medium heat, stirring constantly until boiling. Remove immediately from heat. Immediately add 4 cups of powdered sugar. Using a hand mixer, beat frosting until smooth, add vanilla and salt. Use additional powdered sugar as needed to achieve desired consistency in frosting. Frost bars and serve.