Preheat oven to 350°. Cover two 13x9-in. pans with foil, tightly securing edges around pan. For each pan, cut fifteen small holes in foil, about 2-1/2 in. apart. Carefully stand a cone in each hole.
Prepare cake batter according to package directions. Fill cones to within 1/2 in. of tops (scant 3 tablespoons); do not overfill. Bake until a toothpick inserted in center comes out clean, 17-20 minutes. Cool completely on wire racks.
To serve, pipe or dollop whipped topping over tops to resemble ghosts. Decorate with black icing. Refrigerate leftovers.