Preheat oven to 350°F. Grease and flour two 8-inch round cake pans. Prepare cake mix according to package directions, folding 1/2 cup of graham-cracker crumbs into batter. Bake until golden, 30-35 minutes. Cool cakes completely in pans on wire racks.

Heat whipping cream in a medium saucepan over low heat until bubbles form around the edge of the pan, about 2 minutes. Remove from heat. Add chocolate; stir until melted. Cool until lukewarm, about 5 minutes.

Place marshmallow creme in a microwave-safe measuring cup or small bowl. Microwave on MEDIUM until softened, about 30 seconds. Stir until smooth. Place 1 cake layer on serving plate. Spread top with marshmallow creme, letting sum run down the side. Top with second cake layer.

Pour chocolate mixture over top of cake, letting some run down side of cake. Arrange marshmallows evenly around edge of cake. Sprinkle remaining graham-cracker crumbs in the center of cake. Chill cake at least 30 minutes to set chocolate before serving.