Rub flour and butter to breadcrumb texture.
Add icing sugar, egg yolk, and water to form a dough.
Wrap and chill 20 minutes.
Roll out and line a loose-bottom tart tin (approx. 23cm).
Blind bake at 180°C for 15 minutes, then 5 minutes uncovered.
Heat cream and milk until steaming (not boiling).
Pour over chopped chocolate and butter; let sit 1 minute.
Stir until glossy.
Add orange zest and liqueur.
Pour into the baked shell.
Chill for 2 hours to set.
Serve with: crème fraîche or vanilla ice cream.
