Prepare and bake the chocolate cake according to package instructions in a 9x13-inch pan. Let cool for 5 minutes.
Using the handle of a wooden spoon, poke holes all over the cake.
In a large bowl, whisk together the chocolate pudding mixes and milk until slightly thickened.
Pour the pudding over the cake, ensuring it fills the holes. Refrigerate for 1 hour.
Spread mini marshmallows evenly over the top.
Microwave the Cool Whip and hot fudge sauce together for 15–20 seconds and stir. Spread over the cake.
Sprinkle with mini chocolate chips and marshmallow bits.
Chill until ready to serve. Best served slightly chilled or at room temperature.