Stir together vodka, liqueurs, and, if desired, half-and-half in a large pitcher; chill at least 1 hour.
Fill martini glasses with ice. Let stand 5 minutes; discard ice.
Dip rims of chilled glasses in chocolate liqueur; dip in cocoa, coating rims.
Pour vodka mixture into glasses. Serve immediately. Prep: 15 min.; Stand: 5 min., Chill: 1 hr.
Note: For testing purposes only, we used Godiva Liqueur for chocolate liqueur, Chambord for raspberry liqueur, and Ghirardelli Sweet Ground Chocolate and Cocoa for sweetened cocoa.
Individual Chocolate Martini: Combine 1/4 cup vodka, 2 tablespoons chocolate liqueur, 1 1/2 teaspoons raspberry liqueur, 6 ice cubes, and, if desired, a dash of half-and-half in a martini shaker. Cover with lid, and shake until thoroughly chilled. Remove lid, and strain into a chilled martini glass. Serve immediately. Makes 1 serving.