1. Preheat oven to 350ºF and spray a 9"x13" pan with cooking spray. Prepare cake batter according to package instructions, then fold in crushed graham crackers.
2. Bake until a toothpick inserted in the center of the cake comes out clean, about 30 minutes.
3. Let the cake cool slightly, then poke holes all over the bottom of the cake with a wooden spoon. In a small bowl, mix together sweetened condensed milk and melted chocolate. Pour this mixture into the poked holes.
4. Make the frosting: In a large bowl using a hand mixer, beat softened butter until light and fluffy. Add marshmallow crème, powdered sugar, and vanilla extract and beat until smooth. Gradually add heavy cream if the mixture is too stiff.
5. Frost the cooled cake and top with mini marshmallows, broken chocolate bars, and additional crushed graham crackers. Drizzle with melted chocolate if desired.
