There are 2 ways I know to make good chocolate martini's. You can use Kremyovskya Chocolate Vodka or you can mix vodka and Creme de Caco. Using either will give you a perfectly clear chocolate martini. Using Godiva or other colored cocolate liqueurs will only give you a murky, nasty looking drink.
There is the ever popular "Dirty Martini", vodka and olive juice. You can also make a variety of flavored martini's using popular liqueurs such as cinnamon, berry, orange, peach, ect. Using liqueurs is easy and makes colorful drinks.