For the cake:
Preheat oven to 375° F.
Butter and flour a 15” x 10” pan.
In a large bowl combine flour, sugar, cinnamon and salt.
In a medium saucepan combine water, oil, butter and cocoa. Bring to a boil over medium heat. Pour the chocolate mixture over the dry ingredients and mix thoroughly. Add buttermilk, eggs, vanilla and baking soda and mix thoroughly. Pour batter into prepared pan.
Bake for 15-20 minutes or until a toothpick inserted in the center comes out clean. Place pan on a wire rack.
For the Icing:
In a medium saucepan, combine the butter, cream, cocoa, and cinnamon. Cook over medium heat, stirring constantly until butter is melted and mixture is heated through.
Remove from heat and beat in sugar and vanilla until mixture is smooth.
Working quickly, spread the frosting over the warm cake. Cool completely.