To make the shavings, simply use your cheese grater (use a grater with holes that make a fine shred, not a coarse one). You will get a more light, powdery chocolate if the bar is chilled first.
Use a very good quality chocolate. No Hershey allowed in this!
I recommend Godiva's White Chocolate liqueur.
You can reverse these ingredients and use Godiva's regular Chocolate liqueur, garnished with White chocolate shavings.
Follow these simple steps to make your own White Chocolate Martini: