In a saucepan mix cola, oil, butter and cocoa and bring to boil. In bowl combine sugar, flour and salt. Pour the Cola mixture over the flour mixture and beat well.
Add eggs, buttermilk, baking soda and vanilla and beat well. Pour mixture into a greased and floured 13x9 inch baking pan. Bake at 350 degrees for 20 to 25 minutes.
Frosting
In a saucepan, combine butter, cocoa, and milk. Heat until butter melts. Beat in remaining ingredients and spread on cake while still warm