Preheat your oven according to the instructions on the box mix. Prepare the cake batter as directed and pour it into a greased 9x13-inch baking dish.
Bake the cake according to the package directions, typically 25–28 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool for about 10 minutes. Then, using the handle of a wooden spoon or a thick straw, poke holes all over the top of the cake—aim for about 50 holes to ensure even coverage.
Add the Marshmallow Layer: Scoop the marshmallow fluff into a microwave-safe bowl and microwave it for 30 seconds to 1 minute, or until it’s smooth and pourable. Pour the warm marshmallow fluff evenly over the cake, making sure it seeps into all the holes.
Place the cake in the refrigerator to cool completely while you prepare the whipped cream topping.
Whip the Cream: In a chilled mixing bowl, add cold heavy whipping cream, chocolate pudding, powdered sugar and vanilla extract. Using a hand mixer or stand mixer, beat the cream until stiff peaks form.
Top and Garnish: Spread the whipped cream evenly over the chilled cake. Sprinkle chopped chocolate bars and chopped graham crackers on top for that classic s'mores flavor.
Slice, serve, and enjoy! Store leftovers covered in the refrigerator.
