Chocolate Sheet Cake

I thought I already posted this recipe but I can't seem to find it so maybe not.  This recipe came from my Aunt Nancy and my Mom.  I remember having this at Thanksgiving time over the years when I was young.  Both my mother and her sister married and both had 7 children.  So between 18 of us kids, big and small, this never lasted long.  I used to make this for bake sales.  It is extremely appetizing and the icing, addicting!  I would say the most important thing is that you need a sturdy sheet pan.  If yours twists as it heats up, this cake will not work and the icing will be a disaster.  I have true 1/2 sheet pans which I got from a restaurant supply store but I also have 1 the same size from the Martha Stewart line of bakeware.  They are sturdy and have straight sides and they are light in color without a non stick coating.
Yesterday my nieces S and E were here.  Whenever they are here we frequently bake something together and this was perfect because I could have 3 kids doing 3 different tasks without competing.  One last note, this is the only recipe I tried to make with butter and it didn't work.  Use Margarine for this one.
Preheat oven to 400.  Grease a sheet pan with Crisco shortening.  In a saucepan, add 2 Sticks Margarine, 1 Cup Water, 5 Tbl Cocoa.  Bring to a boil, stirring occasionally.  In the meantime, measure into a mixing bowl, 2 Cups Sugar and 2 Cups Flour.  Add hot chocolate mixture to dry ingredients and mix on medium speed.  Add 1 tsp Baking Soda and mix again.  In a measuring cup add 1/2 Cup Buttermilk, 2 Eggs and 1 tsp Vanilla.  Using a fork, stir together.  Add egg mixture to batter and mix until a smooth batter.  Pour into sheet pan and place in oven on a middle shelf.  Bake for 20 minutes.  Clean the saucepan cause you'll need it for the icing.  With 10 minutes left to bake, make icing.  In saucepan bring to boil, 1 stick Margarine, 3 Tbl Cocoa, 6 Tbl Whole Milk.  Once it boils, add 1 Lb Powder Sugar all at once, a 1 tsp Vanilla and stir until smooth.  Pour over hot cake.  Let it set up at least 1 hour before cutting.  Serve with cold milk!