Prepare the cake in a 9x13-inch pan according to the instructions on the box. Let the cake cool completely to room temperature.
Using the handle of a wooden spoon or the thick end of a chopstick, poke evenly spaced rows of holes into the cake, about 1 inch apart.
In a microwave-safe bowl, combine the marshmallow fluff and water. Microwave for 20 seconds and stir until smooth. The mixture should be pourable—if it’s still too thick, microwave for an additional 10 seconds and mix again.
Pour the marshmallow creme evenly over the cake, using a rubber spatula or the back of a spoon to spread it and gently press it into the holes.
Warm the chocolate fudge sauce according to the jar’s directions, then spread it evenly over the top of the cake. Let the fudge sauce cool completely.
In a medium bowl, combine the whipped topping and hot chocolate mix by gently folding them together until fully blended.
Spread the chocolate whipped topping over the cake and refrigerate for at least 1 hour to allow the flavors to meld together.
Before serving, sprinkle the mini marshmallows and mini chocolate chips on top. Slice and enjoy!