For poached quince, preheat oven to 130C. Combine sugar, vanilla bean and seeds and 1.5 litres water in a saucepan, bring to the boil over medium heat, stir to dissolve sugar. Add quince, cover with peelings, then baking paper, bring to a simmer, cover with a lid, transfer to oven and bake until deep pink (4-6 hours).
Meanwhile, for chocolate pastry, process flour, icing sugar and cocoa in a food processor until combined. Add butter, process until mixture resembles fine crumbs, then add egg yolks, process to combine. Turn onto a lightly floured surface and bring pastry together with the heel of your hand. Wrap in plastic wrap and refrigerate to rest (1 hour).
Preheat oven to 180C. Roll pastry into a 32cm round on a lightly floured surface and line a 24cm-diameter, 4cm-deep buttered and floured tart tin. Refrigerate to rest (1 hour), blind bake until pastry is almost cooked (8-10 minutes), then remove weights and paper and bake until cooked through (8-10 minutes).
Meanwhile, whisk eggs, yolks and sugar in an electric mixer until pale (4-5 minutes). Melt chocolate and butter in a heatproof bowl over a saucepan of simmering water (4-6 minutes). Fold through egg mixture, then pour into hot tart base and bake until set (15-20 minutes). Set aside to cool to room temperature.
Meanwhile, remove quince from syrup and set aside. Strain syrup into a clean saucepan and reduce by half over medium heat (20-30 minutes). Cool slightly, return quince to syrup and serve quince and syrup to taste with tart. Remaining quince will keep refrigerated in a sterilised container for 1 month.
Note This tart is adapted from a recipe from
London restaurant Le Gavroche. If you're short of time, you could
use a good-quality shop-bought chocolate pastry such as Carême's