Grease a 23cm fluted tart tin with butter.
Pulse flour, icing sugar, and butter in a food processor until it resembles breadcrumbs, then mix with egg yolk and 15ml water to form dough.
Wrap the dough in cling film and chill in the fridge for 30 minutes.
Preheat the oven to 200°C.
Roll out the pastry on a floured surface to a circle 5cm larger than the tin.
Line the tin with pastry and chill for 15 minutes.
Bake the pastry blind with weights for 10 minutes at 200°C, then remove weights and bake for 5 more minutes.
Melt butter and dark chocolate over a water bath, then stir in sugar, flour, and four eggs.
Melt butter and white chocolate, then stir in orange zest, sugar, flour, and two egg yolks.
Pour dark chocolate mixture into the shell and marble the orange mixture through using a cocktail stick.
Bake for 10 minutes until set at the edges but wobbly in the centre.
Allow to cool slightly before serving.