In a saucepan, mix the Coca Cola, oil, butter, and cocoa. Bring it to a boil.
In another bowl, combine the sugar, flour, and salt.
Pour the boiling Cola mixture over the flour mixture and mix well.
Add the eggs, buttermilk, baking soda, and vanilla. Beat until smooth.
Pour the batter into a greased and floured 13 x 9 inch baking pan.
Bake at 350 degrees for 20 to 25 minutes.
Let it cool for about 10 minutes before adding the frosting.
For the frosting:
In a saucepan, mix together the butter, cocoa, and milk until melted.
Beat in the remaining ingredients until smooth.
Spread the frosting on the cake while it’s still warm.