Decorate cakes. Melt white candy melts candy according to package directions. Use white melted candy in cut parchment bag to outline and fill in shamrock shape. Gently tap cakes to smooth and remove bubbles. Chill to set, 10 to 15 minutes.
For nonpareil outline shamrock, use white melted candy in cut parchment bag to pipe shamrock outline. Immediately sprinkle with St. Patrick's Day nonpareils. Tap gently to remove excess.
Chill to set, 10 to 15 minutes.