How to Make It
Preheat oven to 350°F. Grease and flour a 9-inch round cake pan; set aside.
Combine cake mix, water, oil, and eggs in a large bowl. Beat with an electric mixer on low speed until combined, about 1 minute. Increase speed to medium, and beat until almost smooth, about 2 minutes. Scrape batter into prepared pan. Bake in preheated oven until a wooden pick inserted in center comes out clean, 30 to 35 minutes.
Cool cake in pan on a wire rack 15 minutes. Remove cake from pan, and cool completely on rack, about 30 minutes.
Combine frosting and powdered sugar in a large bowl. Beat with an electric mixer on medium speed until combined and smooth, about 1 minute. Place 1/4 cup of the frosting mixture in each of 2 small bowls. Add red food coloring to 1 small bowl; leave second small bowl of frosting mixture white. Add black food coloring to remaining frosting mixture.
Using a 3 1/2-inch round cutter, cut rounds from bottom edge and interior of cake, leaving outer edge of cake to form an arc. Place black frosting in a piping bag fitted with a star tip; decoratively pipe black icing over arc to create bat wings. Cut 1 (3 1/2-inch) cake round into a bat head shape. Place on top center of arc, and pipe black icing over bat head. Place red frosting in a second piping bag, and pipe eyes onto bat head. Place white frosting in third piping bag, and pipe a mouth and teeth onto bat head. Reserve remaining cake for another use.