Bake a boxed cake in a 9-by-13-inch pan and let cool. Remove from the pan, then trim off the domed top with a long-serrated knife and slice the cake crosswise into three rectangles (to make three "steaks"). Frost the tops and sides with chocolate frosting, then coat in ground chocolate graham crackers. Flip each cake onto a cutting board; frost the bottom and coat with more crumbs. Use black decorating gel to pipe grill marks. Freeze the cakes for at least 1 hour, then thinly slice. Sprinkle with coarse sugar.