Roll out the pastry on a lightly floured surface until large enough to line a buttered 24 cm (9½ inch) fluted, loose-bottomed flan tin. Lift over a rolling pin, drape into the tin and press over the base and sides. Trim off the excess pastry, prick the base with a fork, then chill for 15 minutes. Line with nonstick baking paper, add baking beans and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 15 minutes. Remove the paper and beans and bake for a further 5 minutes. Remove from the oven and reduce the oven temperature to 160°C (325°F), Gas Mark 3.
Melt the dark and white chocolate in separate bowls over a saucepan of simmering water. Add three-quarters of the butter to the dark chocolate and the rest to the white chocolate. Leave until melted.
Whisk the eggs, egg yolk and sugar in a third bowl for 3–4 minutes until doubled in volume (but not so thick as to leave a trail). Fold two-thirds into the dark chocolate mixture, then pour into the cooked tart case. Fold the cream into the white chocolate to loosen it, then fold in the remaining whisked egg mixture. Spoon over the dark chocolate layer to completely cover. Cook the tart for 20 minutes until just set with a slight wobble to the centre. Cool for at least 1 hour. Pipe double lines of melted dark chocolate and leave for at least 30 minutes before serving.