Preheat oven to 190ºC. Roll out the pastry on a work surface lightly dusted with flour to 5 mm (1/4 inch) thick and use to line a 24cm tart tin with a removable base. Trim excess and prick the base with a fork. Chill for 15 minutes in the freezer or 30 minutes in the refrigerator.
Line the pastry with baking paper and fill with baking beads, dried beans or uncooked rice. Bake for 10 minutes, then remove the paper and beads and bake for another 5 minutes or until light golden and dry. Cool in the tin.
Reduce the heat to 180ºC. Melt the chocolate and butter in a large heatproof bowl set over a saucepan of boiling simmering water. Stir until smooth and incorporated.
Remove from the heat and whisk in the sugar, eggs and vanilla extract, then the flour. Add the nuts and combine well.
Pour into the pastry case and and bake for 30 minutes or until the top is crusty.
Cool in tin. Dust with icing sugar if you like and serve with creme fraiche or ice cream.