Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. In a small bowl, combine graham crumbs with melted shortening. Stir together until mixture resembles coarse crumbs. Mix in chocolate pieces and walnuts; set aside.
Prepare cake mix according to package directions. Spread batter into prepared pan. Sprinkle crumb mixture over batter evenly.
Bake in the preheated oven for 40 to 50 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool in pan.