Preheat the oven to 350 degrees.
In another bowl, stir the cocoa, Coco-cola, butter, and marshmallows. And then, boil the coco-cola mixture. Pour the boiled mixture into the flour and sugar mixture.
Whisk the egg, buttermilk, baking soda, and vanilla in another bowl. And then, combine the egg mixture with the previous mixture.
Pour the mixture into the greased pan.
Place into the preheated oven and bake it for 20 to 25 minutes.
Remove cake out from the oven.
For the frosting, stir the butter, cocoa, and Coco-cola to a boil in a saucepan. And then, add in the sugar and nuts.
While both cake and frosting are still warm, spread over the cake.
Let it cool completely and slice to serve.