In a saucepan, mix Coca Cola, oil, butter and cocoa and bring to a boil. In another bowl, combine the sugar, flour and salt. Pour the boiling Cola mixture over the flour mixture and beat well. Add the eggs, buttermilk, soda and vanilla and beat well. Pour mixture into a greased and floured 13 x 9 inch baking pan and bake at 350 degrees or 20 to 25 minutes. Remove pan. Cool for about 10 minutes before frosting.
In a saucepan, combine the butter, cocoa, and milk. Heat until the butter melts. Beat in the remaining ingredients and spread on the cake while it’s still warm.