Preheat oven to 350°F and grease a 9x13-inch baking dish.
Mix Coca-Cola and buttermilk in a measuring cup.
Beat butter until creamy using an electric mixer.
Gradually add sugar to the butter and mix well.
Beat in eggs and vanilla until blended.
In a separate bowl, combine flour, cocoa, and baking soda.
Add flour mixture to butter mixture in 3 parts, alternating with buttermilk mixture in 2 parts.
Stir in marshmallows, then pour batter into the prepared baking dish.
Bake for 30 to 35 minutes, then cool for 10 minutes before adding icing.
For the icing, melt butter, Coke, and cocoa in a saucepan over medium heat.
Remove from heat, whisk in confectioners' sugar and vanilla until smooth.
Pour icing over cooled cake and enjoy!