Preheat oven to 350 degrees and set out a 9x13 baking dish.
In a large bowl, add all the "cake" ingredients (cake mix, one instant pudding mix, eggs, vanilla, hot chocolate (or chocolate milk), vegetable oil, and sour cream) and beat on medium speed until well combined.
Pour into the 9x13 baking dish and bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Let cool for approximately 10-15 minutes then poke holes all over it using the end of a wooden spoon or something of similar size & shape.
Pour 1 cup of hot fudge sauce into a bowl and warm for approx. 20 seconds. Pour it over the cake and let it absorb into the holes. I spread mine a bit with a spatula. Pop cake into the fridge to completely cool.
Once the cake has cooled, prepare the pudding layer. In a medium bowl, add the instant chocolate pudding mix and 2 cups of milk. Whisk together and immediately pour over the cooled cake. Put the cake back in the fridge to set.
While the cake is setting, put the bowl of your stand mixer (or a metal mixing bowl) and whisk attachment into the freezer to get them really cold, for about 15 minutes.
To make the whipped cream
Remove the mixing bowl and whisk from the freezer. Pour in the heavy cream and beat on medium speed for about 3 minutes. Slowly pour in the chilled hot chocolate (or chocolate milk) while still mixing. Sprinkle in the cocoa powder and sugar, turn the mixer up to medium-high and beat until stiff peaks form.
Remove the cooled cake from the fridge and top with the whipped cream, spreading it evenly with a spatula.
Drizzle with remaining hot fudge sauce and top with marshmallow bits and mini chocolate chips.
Notes
Recipe adapted from Beyond FrostingIf you can't find International Delight Hot Chocolate, chocolate milk can be used.Store cake covered in the fridge.