Step 1

Preheat oven to 350 degrees and spray a 9x13-inch baking dish with baking spray.

Step 2

Combine Coca-Cola and buttermilk in a glass measuring cup. Set aside.

Step 3

Using an electric mixer (either stand or hand-held) beat butter until creamy.

Step 4

Gradually beat in sugar.

Step 5

Add eggs and vanilla and beat on low speed until blended.

Step 6

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.

Step 7

Stir in marshmallows. Pour batter into prepared baking dish.Bake for 30 to 35 minutes. Let cool 10 minutes before adding icing.

Step 8

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.