Smash the tangerine zest with the sugar to moisten the sugar with the fruit’s aromatic oils. Transfer to a 1-quart saucepan along with the cornstarch, juice, and salt. Stir to dissolve the cornstarch. -- Turn on the heat, bring the mixture to a boil, and cook, stirring, until the juice has thickened, after just a few minutes. Cook for 1 minute more, then turn off the heat and whisk in the butter and yuzu or orange-flower water. Taste, and if extra sweetener is needed, add a few drops of stevia, orange blossom honey, or agave nectar. -- Divide among juice glasses or Champagne glasses. Refrigerate until set, about 2 hours. (If you’re not counting calories too carefully, this pudding is great with a dollop of whipped cream.)