Combine 2/3 cup boiling water and chopped chocolate in a medium saucepan, stirring until chocolate melts. Add milk and next 4 ingredients (through rind); cook over medium-low heat, stirring with a whisk. Heat 5 minutes or until tiny bubbles form around edge of pan, stirring frequently (do not boil). Discard rind. Pour 1 cup mixture into each of 4 mugs. Spoon 1 tablespoon whipped topping over each serving. Dust with cocoa powder, if desired.