To make the cake, put the Coca-Cola, butter and cocoa in a saucepan and bring to a boil over medium heat. Just after it begins to boil, remove from heat and let cool for about 10 minutes.
In a separate bowl, mix flour, sugar, buttermilk, baking soda, eggs and vanilla. Mix until well combined.
Add the cooled cocoa mixture to the flour mixture and mix well. Add the marshmallow creme, stir until well combined.
Pour mixture into a greased Bundt pan and bake at 350° for about 45-50 minutes or until toothpick inserted in the center comes out clean. (If using a 9X13 pan, cook for about 30 minutes.)
To make the icing, start by melting the butter In a saucepan over medium heat. Add the milk and sugar and bring to a boil, stirring constantly. Boil for one minute and then remove from heat. Stir in the chocolate chips until completely melted and smooth. If you let the frosting sit for just a minute or two, it will start to set up a little bit, making it the perfect consistency to pour/spread on the cake. Enjoy!