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Preheat oven to 350 degrees and spray a 9x13-inch baking dish with baking spray.
Combine Coca-Cola and buttermilk in a glass measuring cup. Set aside.
Using an electric mixer (either stand or hand-held) beat butter until creamy.
Gradually beat in sugar.
Add eggs and vanilla and beat on low speed until blended.
In a bowl, combine flour, cocoa, and baking soda.Add flour mixture to butter mixture in 3 additions, alternating with buttermilk mixture in 2 additions. Beat just until blended.
Stir in marshmallows. Pour batter into prepared baking dish.Bake for 30 to 35 minutes. Let cool 10 minutes before adding icing.
To make icing, combine butter, coke, and cocoa in a large saucepan and place over medium heat until butter is melted, stirring frequently.Remove from heat. Whisk in confectioners' sugar and vanilla. Pour on top of cake.