Combine margarine, cocoa, water, and shortening in a heavy saucepan. Bring to a boil, stirring occasionally. Then add the flour, sugar, soda, buttermilk, cinnamon, vanilla, and the eggs. Remove from heat and beat with mixer until mixed well, Pour into a greased and floured 11x16 inch cookie sheet. Bake at 400F for 15 to 20 minutes.
Combine all ingredients in a saucepan over low heat until well mixed. Cool a little and spread icing over warm cake.
Cajun Country -- New Iberia, Louisiana