In a saucepan, bring coca-cola, margarine, cocoa, and oil to a boil.
In a separate bowl mix sugar, flour and salt then pour into the boiling mixture and mix well.
Add eggs, buttermilk, vanilla, and soda and mix well. Pour into a greased and floured sheet cake pan (9x13) and bake at 350 degrees for 20-25 minutes.
For Frosting: In saucepan combine margarine, cocoa, and milk and heat until the butter melts. Beat in the remaining ingredients and spread onto HOT cake. Cool and cut.