To make these treats, cut a prebaked pound cake into 1-inch slices. Cut each slice with a shamrock-shaped cookie cutter. Microwave an open container of vanilla frosting just until you can pour it, about 10 seconds, and tint green with food coloring. Place shamrocks on a wire rack over a sheet of wax paper. Pour frosting over shamrock slices. Sprinkle edges with green crystal sugar. Let shamrocks set 5 minutes before transferring to serving plate.