Prepare and bake cake mix in a 9x13 pan according to package directions. Once cake is done baking, let cool while you prepare the s'more toppings.
Remove the lid and liner from the marshmallow cream packaging and microwave for 30 seconds. Transfer to a small bowl and stir in the 1/4 cup of milk. Microwave for another 30 seconds and stir until smooth.
Using the handle of a wooden spoon, poke holes in the cake at about 1 inch intervals. Pour marshmallow cream mixture over cake, doing your best to pour over the holes. You can use a rubber scraper to even out the marshmallow cream over the cake.
Sprinkle with crushed graham crackers. Drizzle some of the melted chocolate on top of the graham crackers, then sprinkle the mini marshmallows on top. Pour the rest of the melted chocolate on top of the marshmallows. Finish by sprinkling the mini chocolate chips on the very top. Store cake in the fridge for about an hour before serving to allow the marshmallow cream to seep down into the cake. Refrigerate any leftovers.