1Heat oven to 350°F. Line 13x9-inch pan with foil, leaving foil overhanging at 2 opposite sides of pan. Make and bake brownie mix as directed on box for cakelike brownies, using water, oil and eggs. Cool completely in pan on cooling rack, about 1 hour. Freeze 10 minutes.
2Use foil to lift brownies out of pan and invert onto work surface. With 5x2 1/2-inch coffin-shaped cookie cutter, cut 8 coffins from brownies. (Rotate cookie cutter top to bottom and then bottom to top, making cutouts as close together as possible.) Cut each coffin in half horizontally with serrated knife. For each coffin, spread about 2 tablespoons frosting on bottom half of brownie. Arrange 8 bone sprinkles and 1 candy skull on frosting to look like skeleton inside coffin. Outline top of each coffin with white icing. Pipe RIP in center with icing. Place coffin tops, cut sides down and slightly off center, on coffin bottoms.
3Sprinkle cookie crumbs on serving platter to look like dirt. Place coffins on crumbs. Serve or refrigerate.