Preheat your oven to 350 degrees.
In a large pot, bring the butter, Coca-Cola and cocoa powder to a boil whisking until combined. Remove from heat.
In a separate bowl, combine the flour, sugar, mini marshmallows and salt.
Quickly add the flour mixture into the pot and mix until combined and marshmallows are almost melted. It's okay if some marshmallows are not completely melted.
In a liquid measuring cup, whisk together eggs, buttermilk, baking soda and vanilla. Add into the pot and continue to fold the mixture until everything is combined.
Line a baking sheet with parchment paper and rub with butter. Pour the batter into the pan.
Bake in a 350 degree oven for about 25 minutes. Remove before the center is fully cooked! You don't want a toothpick to come out clean... you want it to come out with a little cake on it.
Let it cool completely, then place it upside down on a platter until the cake tips out.