Chocolate meringue pie

Method

1. For the pastry, pulse together the sugar and butter in a food processor until combined. Add the flour and cocoa powder and pulse to a breadcrumb-like texture.
2. Add the egg yolk and 2 tbsp cold water, then pulse to bring the dough together – you may need a little more water, but be careful not to make the dough too wet. (If you don’t have a food processor, beat the sugar and butter with a wooden spoon, then rub in the flour and cocoa powder with your fingertips. Use the spoon to mix in the egg yolk and water, then knead together.)
3. Once the dough is smooth, shape it into a thick disc, wrap in cling film and chill for 30 minutes.
4. Roll out the pastry between 2 sheets of baking paper to the thickness of a £1 coin. Peel off one piece of baking paper, then gently roll the pastry around the rolling pin to lift. Unroll the pastry, baking paper-side up, over a 23cm, deep fluted tin, then peel off the remaining paper (set aside). Carefully press the pastry into the tin. Heat the oven to 200°C/fan180°C/gas 6.
5. Trim any excess pastry from the sides with a sharp knife and use it to patch any gaps or holes. Scrunch up the reserved baking paper, then flatten out and use it to line the pastry case (scrunching helps it fit into the corners). Fill with baking beans/rice, then chill for 15 minutes. Bake for 13 minutes, then remove the baking beans/rice and cook for a further 2 minutes or until the pastry feels sandy and dry.
6. Meanwhile, for the filling, put the chopped chocolate into a heatproof bowl over a pan of barely simmering water (don’t let the water touch the bowl). Melt gently, stirring now and then, then add the butter and stir until combined. Take off the heat.
7. Using an electric mixer (or balloon whisk) whisk the sugar, eggs and egg yolks until pale and creamy. Stir in the melted chocolate until it’s all incorporated, then gently fold in the flour using a large metal spoon.
8. Lower the oven temperature to 180°C/fan160°C/gas 4. Spoon the chocolate filling into the cooked pastry case, level the top with a spatula, then bake for 12 minutes.
9. Meanwhile, make the meringue. Using an electric mixer with clean beaters (or a clean balloon whisk), whisk the egg whites (make sure the bowl is clean too) until they form stiff peaks. Gradually whisk in the sugar until you have a stiff, billowy meringue whose peaks stay standing when you lift out the beaters/whisk. Mix/whisk in the cornflour, vanilla bean paste and vinegar.
10. After 12 minutes of cooking, remove the chocolate tart from the oven. Lower the oven temperature to 170°C/fan150°C/gas 3½.
11. Sift half the cocoa powder on top of the meringue then, using a metal spoon, carefully spoon the meringue on top of the chocolate tart, swirling slightly as you go. Do not over-stir or you will lose the swirl effect. Sift the remaining cocoa over the meringue.
12. Return the tart to the oven and bake for 20 minutes more or until it has a thin crust on top but is still soft in the centre. Leave to cool completely before removing from the tin, then slice to serve.