1. Heat zest, half the juice, the sugar, and salt together in a small saucepan over medium heat until simmering and sugar has dissolved, about 5 minutes. Add remaining tangerine juice, stirring to combine. Transfer mixture to a bowl and chill until cold, about 30 minutes. Stir in half-and-half and, if you like, liqueur.
2. Freeze in an ice cream maker according to manufacturer's directions. Transfer to a sealable container and freeze until firm, at least 4 hours.
Make ahead: Up to 1 week.