In a small heavy saucepan cook and stir dark chocolate and whipping cream over low heat until chocolate is melted and mixture is smooth. Transfer chocolate mixture to a medium mixing bowl. Cool for 20 to 30 minutes. Meanwhile, line a tray or baking sheet with parchment paper; set aside.
Add liqueur to chocolate mixture. Beat with an electric mixer on medium speed for 1 minute. Transfer mixture to a pastry bag fitted with a large star tip. Pipe into ten 2-inch mounds onto the prepared tray. Top each mound with candied orange zest, if desired. Cover loosely and chill for 1 to 24 hours. Sprinkle lightly with cocoa powder before serving.