In a small saucepan heat and stir chocolate over low heat until melted. Cool slightly. Pipe or brush stripes of melted chocolate onto the insides of four 8-ounce glasses. Chill in the refrigerator about 15 minutes or until chocolate sets.
To serve, spoon small scoops of ice cream into each glass, alternating scoops of vanilla and chocolate ice cream. In a small pitcher or 4-cup glass measuring cup stir together milk and liqueur; slowly stir in carbonated water. Pour into glasses over ice cream. Serve immediately. Makes 4 servings.