1. Heat oven to 250 degrees F with two racks centered. Line two baking sheets with parchment paper. Combine egg whites, sugar and cream of tartar in a heat-proof bowl. Place over a pot of simmering water and whisk gently until sugar is dissolved, about 3 min. Remove from heat. With an electric mixer, whisk on medium-high until a stiff meringue forms, about 10 min.
2. Transfer meringue to a piping bag fitted with a #12 plain tip, then pipe 3-1/2- to 4-inch-long bone shapes on baking sheets. Bake 20 min. Turn oven off and let meringues dry in oven for 30 min more. Let bones cool before carefully removing from parchment paper.
3. Coat one cake layer with 1/2 cup frosting and top with second layer. Frost entire cake and refrigerate to set, 30 min or overnight. Decorate with whole and broken bones before serving. Makes 12 servings.
1. In a mixing bowl, beat butter and cream cheese together until creamy, about 2 min. Stir in cocoa. Add confectioners' sugar, 1 cup at a time, mixing well. Beat in vanilla and milk and stir until smooth. Can be refrigerated up to one week; bring to room temperature and stir before using Makes 3 cups.