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.
Whisk together flour, sugar and baking soda in a large bowl. Set aside.
Preheat oven to 350 degrees. Prepare 9x13 baking pan with butter and flour or baking spray.