Chocolate Buttermilk Sheet Cake

Ingredients:
Cake: 2 cups of sifted flour 2 cups sugar 1/2 cup butter 1/4 cup cocoa 1 cup cold water 1/2 cup buttermilk 1 teaspoon vanilla 1 teaspoon cinnamon 1/4 teaspoon salt 1 teaspoon baking soda  2 eggs Frosting: 1/2 cup butter 1/4 cup cocoa 5 tablespoons buttermilk 4 cups icing (confectioners') sugar  (sifted, you don't want white lumps  of sugar in your frosting!) 1 teaspoon vanilla
Directions:
Cake: Preheat oven to 400 degrees F.  In a large bowl combine the flour and sugar.  In a small saucepan, or in a microwave dish, combine the butter, cocoa and water.  Heat to boiling. Pour over the flour mixture and mix well.  Add buttermilk, vanilla, cinnamon, salt, baking soda and eggs.  Mix well. Pour batter into a greased jelly roll pan , 15"x10"x1" pan.  Bake for 20 minutes. Frosting: 5 minutes before the cake is done, combine butter,  cocoa and buttermilk in a medium saucepan.  Bring to a boil.  Remove from heat, beat in icing sugar and vanilla.  Beat until smooth.  Remove cake from oven and spread icing while the cake is hot.