Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan. Prepare the cake batter according to the package directions and pour it into the prepared pan.
Bake as directed on the box, typically 25-30 minutes, or until a wooden pick inserted into the center comes out clean. Let the cake cool on a wire rack for 15 minutes.
While the cake is still warm, use the round handle of a wooden spoon to poke holes all over the top, about 1 inch apart. Push down about two-thirds of the way into the cake.
In a medium microwave-safe bowl, combine the marshmallow fluff and water. Microwave in 20-second intervals, stirring each time, until the mixture is smooth and pourable.
Carefully pour the marshmallow mixture over the warm cake, spreading it gently so it sinks into the holes.
Warm the hot fudge topping in the microwave for 30-45 seconds until it is pourable. Reserve about 2 tablespoons for garnish. Pour the remaining fudge over the marshmallow layer and spread evenly.
Refrigerate the cake for at least 2 hours, or until the fudge layer is firm and the cake is completely cooled.
In a separate bowl, combine the thawed whipped topping and the dry hot cocoa mix. Stir until fully combined and no streaks remain. Spread this frosting over the chilled cake.
Drizzle the reserved hot fudge over the top and sprinkle with mallow bits or mini marshmallows before slicing and serving.