Heat oven to 350°F (325°F for dark or nonstick pans). Spray 2 (8- or 9-inch) round cake pans with cooking spray. Line bottoms of pans with cooking parchment paper, if desired, to prevent sticking.
Make and bake cake as directed on box for two 8- or 9-inch round cake pans. Cool in pans 10 minutes. Remove from pans to cooling rack. Cool completely, about 1 hour. Meanwhile, use black and orange gel or icing to write RIP on oval-shaped cookies and miniature candy bars; set aside.
On serving plate, place 1 cake layer, rounded side down. Frost top of cake layer with about 1/3 cup of the chocolate frosting. Top with second layer, rounded side up. Frost side and top of cake with remaining frosting.
Sprinkle chocolate cookie crumbs on top of cake to make dirt. Insert cookies and candy bars into top of cake for tombstones. Insert 1 end of toothpick into bottom of each ghost-shaped marshmallow. Insert other end into top of cake.
Break 8 of the pretzel sticks into thirds. Use whole pretzels and pretzel thirds to make a fence around edge of cake as shown in photo.
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