Chill a martini glass by placing it in the freezer or filling it with ice water for a few minutes.
Drizzle chocolate syrup inside the chilled martini glass and swirl to coat the sides. Return to the freezer to set while you prepare the cocktail.
Fill a cocktail shaker with ice.
Add vodka, chocolate liqueur, creme de cacao, and half-and-half or heavy cream to the shaker.
Shake well until the mixture is chilled, about 15-20 seconds.
Strain the cocktail into the prepared martini glass.
Garnish with shaved chocolate or a dusting of cocoa powder on top.
Enjoy the rich and decadent flavors of this Chocolate Martini!