Preheat oven to 350 degrees F. Lightly oil and dust with flour a 13- x 9-inch baking pan.
In a small bowl, combine coca-cola soda, buttermilk and vanilla extract; set aside.
In medium-sized bowl, sift together the flour, cocoa powder, and baking soda; set aside.
In large bowl, on low speed, beat together the butter and sugar until combined. Add eggs, one at a time, and beat until blended. Starting with the flour mixture, beat in a little at a time alternating with the Coca-Cola mixture until you have ended with the flour mixture. Continue to beat for 2 additional minutes until the batter is light and fluffy. Stir in the marshmallows. Pour the cake batter into the prepared baking dish. It is normal for the marshmallows to float to the top of the cake batter when cooking.
Bake approximately 30 to 35 minutes, rotating the pan halfway through the cooking (once you reach 30 minutes, check cake every few minutes until done). Cake is done when a toothpick inserted in the center of the cakes comes out clean or when the internal temperature registers approximately 205 to 209 degrees F. on your cooking thermometer.
While the cake is baking, prepare the Coca Cola Chocolate Icing. Do not make the frosting in advance, as you you need to pour it over the cake shortly after baking.
Remove cake from the oven to a cooling rack. While the cake is still hot, pour the prepared Chocolate Icing over cake and spread evenly to cover the cake. Let cake cool completely before serving. Serve alone or accompanied by vanilla ice cream.
In a large saucepan over medium heat, stir together the butter, Coca-Cola soda, and cocoa powder until butter has melted. Whisk in the powder sugar, one cup at a time, and then add the vanilla extract. Stir until icing is smooth.
* For buttermilk substitute: In liquid measuring cup, add 1 tablespoon of lemon juice or white vinegar, pour in milk to the 1 cup line. Let stand for 5 minutes then use the amount you need for your recipe.
Coca Cola Chocolate Cake Recipe: https://whatscookingamerica.net/cake/cocacola-chocolatecake.htm