Preheat the oven to 350F (175C) and grease a 9"x13" baking pan
In a saucepan, combine the Coca-Cola, cocoa, and butter. Bring to a boil, then set aside.
In a mixing bowl, combine the flour and sugar. Add the boiled mixture, mixing well.
Add buttermilk, eggs, baking soda, and vanilla, mixing until smooth
Pour the batter into the prepared baking pan
Bake for about 30 minutes
For the frosting, combine butter, cocoa, Coca-Cola, and vanilla in a saucepan, bring to a boil. Then mix in a powdered sugar until smooth.
Pour the frosting over the warm cake