Preheat the oven to 200°C. Roll the pastry out to 1cm thickness on a lightly floured surface. Use it to line a 7 inch springform fluted tart tin. Prick the base and line with nonstick baking paper and baking beans. Blind bake for 15 minutes until the edges just start to colour.
Remove from the oven and discard the paper and baking beans. Return to the oven to brown the base for minutes. Remove after 5 minutes and brush the base with the beaten egg. Return to the oven for 1-2 to let the egg set and form a seal, then remove and transfer to a wire rack to cool.
Meanwhile, beat together the melted butter, melted chocolate, eggs and flour in a large mixing bowl until smooth. Reduce the oven to 180°C. Pour the chocolate filling into the tart and bake for 5-7 minutes until just set and glossy on top. Remove and allow to cool before turning out from the tin and serving on top of a wire rack.