Preheat oven to 350°F (175°C) and prepare the cake according to box instructions using a 9x13-inch pan.
Once baked, use the handle of a wooden spoon to poke holes evenly across the warm cake.
Pour the sweetened condensed milk over the cake, letting it soak into the holes.
Microwave the hot fudge until pourable, then drizzle it over the cake.
Warm marshmallow creme slightly and spread it over the top of the cake.
Sprinkle mini marshmallows and chocolate chips over the cake.
Let cool completely, then spread whipped topping evenly over the top.
Chill for at least 1 hour before serving.