Preheat the oven to 350 F. In a large bowl, mix together cake mix, eggs, and vegetable oil until smooth. Fold in the crushed graham crackers.
Press the cookie dough evenly into a greased 8-inch square baking dish. Bake for 20-25 minutes. Remove from the oven and top with chocolate bars and marshmallows. Bake for another 3-5 minutes.