In a heavy large saucepn, combine the egg yolks, milk, the 1/3 cup sugar, and the espresso powder or coffee crystals. Cook and stir over medium heat until mixture just coats a metal spoon. Remove from heat. Cool quickly by placing saucepan in a sink or bowl of ice water and stirring for 1 to 2 minutes. Stir in the 1/4 cup rum or milk and the vanilla. Cover surface with plastic wrap. Refrigerate for at least 4 hours or up to 24 hours.
At serving time, in a chilled medium mixing bowl, combine whipping cream and the 2 tablespoons sugar; beat with the chilled beaters of an electric mixer on medium speed until soft peaks form (tips curl). Transfer chilled egg mixture to a punch bowl. Fold in whipped cream mixture. If desired, garnish each serving with orange peel curls. Serve at once. Makes 8 servings.