In pan, mix coke, oil, butter, and cocoa and bring to a boil. In separate bowl, combine sugar, flour and salt. Pour boiling coke mix over flour mix and beat well. Add eggs, buttermilk, soda and vanilla and beat well. Pour into greased and floured 9X13 pan. Bake @ 350 for 20 - 25 minutes. Remove and let cool 10 minutes before 10 frosting.