Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking soda, and salt. Whisk together until well mixed.
In another bowl, mix the Coca-Cola, vegetable oil, buttermilk, eggs, and vanilla extract until combined.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined. If using, fold in the mini marshmallows.
Pour the batter into the prepared baking pan and smooth the top. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
While the cake is baking, prepare the frosting. In a saucepan over medium heat, melt the butter. Stir in the cocoa powder and milk, bringing it to a gentle boil. Remove from heat and whisk in the powdered sugar and vanilla extract until smooth.
Once the cake is done, allow it to cool in the pan for about 10 minutes before pouring the frosting over the warm cake. Let it cool completely before serving.