Preheat the oven to 180°C (160°C fan oven) Gas Mark 4. Roll out the pastry on a lightly floured surface until big enough to line a 20cm round loose-bottomed tart tin. Line the pastry with greaseproof paper, fill with baking beans and bake for 25 minutes. Remove the paper and beans and cook for 10 minutes more until sandy and golden. Set aside.
Make the filling by beating the eggs and sugar together with an electric whisk to create a mousse-like texture. This will take a while. Stir in the vanilla or orange extract with the melted butter, then fold in the flour and cocoa. Stir through the chopped chocolate and pour the filling into the tart case. Bake for 20-25 minutes until firm, with a slight wobble in the middle.
Serve just warm or at room temperature – it’s delicious with a dusting of cocoa, a scoop of vanilla ice cream and a dollop of orange curd.