• Preheat the oven to 350˚F. Combine the Coca-Cola and buttermilk; set aside.
• Beat the butter with an electric mixer at low speed until creamy. Gradually add the sugar and beat until blended. Add the eggs and vanilla and beat at low speed until blended.
• Combine the flour, cocoa, and baking soda. Add to the butter mixture alternately with the cola mixture, beginning and ending with the flour mixture. Beat at low speed just until blended.
• Stir in the marshmallows. Pour the batter into a greased and floured 13 x 9-inch pan. Bake in the preheated oven for 30 minutes. The marshmallows will have risen to the top of the cake. Remove from the oven; cool on a wire rack 10 minutes. Pour the Coca-Cola Frosting over the warm cake while still in the pan; garnish, if desired.

Coca-Cola Frosting

• Combine the butter and Coca-Cola in a large saucepan over medium heat, stirring until the butter melts. Remove from the heat; whisk in the cocoa, sugar, and vanilla. Pour warm icing over the cake. Makes 2 1/4 cups.