Heat oven to 350°F. Grease and flour two 8-inch round cake pans. Prepare cake according to package directions. Divide evenly between the two pans. Bake until golden brown, about 20 minutes, or until a toothpick inserted into the center comes out clean. Transfer to a wire rack to cool 10 minutes. Invert, and let cool completely.
Tint the frosting a pale yellow. Place one layer on work surface, and spread with 1/2 cup of frosting. Place other layer on top of frosted layer. Refrigerate cake 30 minutes. Cut a 3/4-inch piece from cake to create a flat edge. Place cake on a serving platter, flat side down, and frost with remaining yellow frosting. Sprinkle with sugar crystals.
Line a sheet pan with wax paper. Place chocolate wafers in a resealable bag, and melt in the microwave for 15-second intervals, until smooth, about 1 minute. Snip a small corner from the bag, and pipe outlines of bats onto wax paper. Fill in with chocolate, and refrigerate until set, about 5 minutes.
Insert toothpicks into marshmallows. Peel chocolate bats from wax paper. Attach a marshmallow to the center of the back of each small bat using a dab of chocolate. Repeat this with the larger bat, attaching two marshmallows to back. Refrigerate bats until set, about 2 minutes. Attach bats to cake by inserting toothpicks halfway into cake.
Remove toothpicks from cake before serving.