Preheat oven to 350 degrees. Prepare 9x13 baking pan with butter and flour or baking spray.
Whisk together flour, sugar and baking soda in a large bowl. Set aside.
Add Coca-Cola, butter and cocoa to medium saucepan. Bring to a boil over medium heat, stirring constantly. Remove from heat and gradually add to the flour mixture with a large spoon. Stir in the marshmallows, eggs, buttermilk and vanilla until well-combined. Pour cake batter into your prepared 9x13 baking pan.
Bake for 30 minutes, or until a toothpick inserted in the center comes out clean. You can follow my tips forĀ how to tell when your cake is done.
Coca-Cola Frosting:
Add butter, Coca-Cola, and cocoa to a medium saucepan. Bring to a boil over medium heat, stirring constantly until the butter completely melts. Remove from the heat and add sugar and vanilla. Stir with a whisk until well-combined and smooth.
Pour over warm cake. Top cake with cocktail peanuts (optional).