Microwave frosting in a microwave-safe bowl at HIGH 30 seconds to 1 minute or until melted; stir until smooth. Let stand 2 minutes.
Place 1 cup melted frosting in a gallon-size zip-top plastic bag. (Do not seal.) Snip 1 corner of bag to make a small hole. Pipe frosting onto each cake square in the shape of a ghost, adding remaining frosting to bag as needed. Pipe 2 dots on each cake using black gel to form eyes.
For testing purposes only, we used Little Debbie Devil Squares and Betty Crocker Decorating Gel.