In a food processor, combine flour, powdered sugar, almond flour, salt, and orange zest. Add butter and pulse until coarse crumbs form. Mix in egg yolk and cold water until dough forms. Wrap and chill for 15-60 minutes.

Preheat oven to 200°C (390°F). Grease tart pan, roll out dough and press into the pan. Chill for 15 minutes. Line with parchment, fill with weights and bake for 12-15 minutes. Remove weights and bake uncovered for 10-12 minutes until golden.

In a saucepan, heat orange juice. Whisk eggs, superfine sugar, and cornstarch in a bowl. Slowly add warm juice while whisking. Cook mixture over medium-low heat, stirring until thickened, then stir in gelatin and cooled butter.

Cool tart shell, pour orange filling into the shell and refrigerate for at least 3 hours, or overnight until set.

Whip cream until soft peaks form. Add sugar and orange bitters if desired. Garnish the tart with whipped cream and crushed pistachios or mint leaves.