Soak prunes, brandy, and vanilla in 50ml boiling water overnight.
Sift flour, icing sugar, and cocoa then rub in butter until it resembles breadcrumbs.
Mix egg yolk, lemon juice, and water into the flour to form a dough ball.
Rest the pastry in the fridge for 15 minutes and preheat the oven to 200°C.
Roll out pastry to line a 23cm tart tin and fill with baking beans.
Bake blind for 15 minutes, remove beans, then bake for 8 minutes until dry.
Trim excess pastry, patch cracks, and reduce oven temperature to 180°C.
Melt chocolate and cream over a bain-marie, then whisk in mascarpone, eggs, and prunes.
Pour filling into the case and bake for 20–25 minutes until almost set.
Cool completely in the tin and serve at room temperature.