Preheat oven to 325 degrees F. In a saucepan, mix water, oil, margarine, and cocoa. Bring to a boil. Cocoa should dissolve. Remove from heat. Add sugar, eggs, and flour. Beat well. Dissolve baking soda in buttermilk. Then pour the mixture and the vanilla into the batter. Mix very well. (If there is lumps from the flour put in the blender for a few minutes) Grab a 10x17 cookie sheet and grease well and then pour batter evenly in. Bake for 20 minutes. (you can add 1/3 cup chopped nuts if you want also) Rinse out the saucepan and then return it to the stove and make the frosting.
Put all ingredients (except the vanilla) in the saucepan. Stir well over medium heat until everything dissolves. Remove from heat and add vanilla. Stir well and then frost while warm.