In saucepan over low heat, stir chocolate pieces, corn syrup and water together until chocolate is melted. Pour into a small pitcher or cup, cover and chill until cool. Add vanilla.
In a large bowl, beat the cream, while gradually adding the chocolate mixture. (Using electric mixer at medium speed.) Continue beating until mixture just mounds; spoon fluffy chocolate whipped cream into a crystal serving bowl and refrigerate. (You can fix this in the morning for the afternoon tea, or an hour ahead.)
Just before serving, scald the milk and pour into a warmed serving pot. Place on tray with cups and saucers and crystal bowl of chocolate whipped cream.
To serve, spoon some of the whipped cream mixture into each cup; fill cups with hot milk. Let guests stir the two together before sipping.