Preheat oven to 350 F. In a large bowl, whisk together flour, sugar, salt and baking soda. In a large saucepan over medium heat, combine cola, butter and cocoa powder. Bring to a boil.
Pour cola mixture into the dry ingredients along with milk, eggs and vanilla extract. Whisk until smooth. Pour cake batter into a large greased 9x13-inch baking dish. Bake for 40-45 minutes.
While the cake bake, for the frosting bring cola to a boil in a large saucepan over medium heat. Add in butter and cocoa powder until smooth. Add in vanilla extract and powdered sugar 1 cup at a time until smooth.
When cake is done baking, poke holes into the cake and pour the cola frosting onto the hot cake. Let cool completely.
