Bake the cake in a 9x13 baking dish as instructed on the cake mix box. Allow cake to come to room temperature

Use the back of a large spoon to poke holes about 1” apart, only poke down about ⅔ of the cake depth.

In a medium microwave-safe bowl, add marshmallow fluff and water. Microwave for 30-second increments, stirring in between until smooth. Fill a piping bag with the marshmallow fluff and cut a hole in the tip. Pipe marshmallow fluff into each hole. Spread any remaining fluff onto the top of the cake. Set in the fridge while preparing the fudge.

In a medium microwave-safe bowl, add the fudge-filling ingredients. Microwave for 30-second increments, stirring in between until smooth and creamy. Spread fudge filling on top of the marshmallow. Set in the fridge to set while preparing the topping.

In a medium bowl, fold together the cool whip and instant chocolate mix. Spread on top of the cake. Drizzle hot fudge sauce and marshmallows on top before serving