Method

1. Grease a 23cm/9in fluted tart tin with butter.

2. For the pastry, measure the flour, icing sugar and butter into the bowl of a food processor and pulse until the mixture resembles fine breadcrumbs. Add the egg yolk and a tablespoon of cold water and mix until it comes together to form a soft dough.

3. Wrap the dough in cling film and leave to rest in the fridge for about 30 minutes.

4. Preheat the oven to 200C(180C fan)/400F/Gas 6.

5. Dust the work surface with flour then roll out the pastry as thinly as you can to a circle about 5cm/2in larger than your flan tin.

6. Line the tin with the pastry. Don’t worry if the pastry breaks a little, it is easy to patch up. Chill for 15 minutes.

7. Prick the base of the pastry with a fork, line the pastry case with baking paper or foil and fill with baking beans. Bake the pastry blind for 10 minutes, or until just lightly golden-brown. Remove the paper and beans and return the tart to the oven to cook for a further 5-7 minutes, or until pale golden-brown and the base is cooked.

8. To make the chocolate filling, melt the butter and chocolate in a heatproof bowl set over a pan of gently simmering water until melted and smooth. Remove from the heat and stir in the sugar and flour. Beat in the eggs, one at a time, and leave to stand.

9. To make the orange filling, melt the butter and white chocolate in a heatproof bowl set over a pan of gently simmering water until melted and smooth. Remove from the heat and stir in the orange zest, sugar and flour. Beat in the egg yolks, one at a time and pour the mixture into a jug.

10. Place the pastry case on a baking tray. Pour the chocolate mixture into the pastry case. Drizzle or pipe the orange filling over the chocolate filling to create a swirl effect. Draw a cocktail stick through the filling to create a marbled effect.

11. Bake for 10-12 minutes, or until just set around the edges, but still slightly wobbly in the centre.

12. Remove from the oven and allow to cool slightly, until warm but not piping hot, then serve.