1. Whisk together first 3 ingredients in a 3-qt. heavy saucepan. Whisk in tangerine juice until smooth. Remove 1 (2-inch) strip of peel from tangerine, using a vegetable peeler, and add to pan. Bring to a boil over medium-low heat, stirring occasionally; boil, stirring constantly, 1 minute or until mixture is thick and bubbly. Remove pan from heat.
2. Whisk eggs until frothy. Gradually whisk 1/4 cup hot juice mixture into eggs; add egg mixture to remaining hot juice mixture, whisking constantly. Return to heat, and cook, stirring constantly, 2 minutes or until thickened. Transfer mixture to a medium bowl; stir in butter and next 2 ingredients until butter is melted. Place heavy-duty plastic wrap directly onto warm custard (to prevent a film from forming); chill 2 hours. (Mixture will thicken as it chills.) Discard peel, if desired. Serve with whipped cream.