Chill bowl, beaters and cream before whipping. Place 2/3 cup cream and sugar in chilled bowl. Beat with electric mixer at high speed until soft peaks form. Do not overbeat. Cover and refrigerate up to 8 hours. If mixture separates slightly after refrigeration, whisk lightly with wire whisk.
To form chocolate shavings for garnish, make short, quick strokes across chocolate with vegetable peeler; set aside. Break remaining chocolate into pieces.
Place remaining 1/3 cup cream in heavy small saucepan. Bring to a simmer over medium-low heat. Add chocolate pieces; cover and remove from heat. Let stand 5 minutes or until chocolate is melted; stir until smooth. Add coffee to chocolate mixture. Heat over low heat just until bubbles form around edge of pan and coffee is heated through, stirring frequently. Remove from heat; stir in creme de cacao, if desired. Pour into 4 warm mugs. Top each mug with whipped cream. Garnish with chocolate shavings.