Photograph: Jenna Leiter
Place the flour, unsalted butter and a pinch of salt in a food processor, pulse until it reaches breadcrumb stage. Add the sugar and egg yolk, pulse, then add 1 tbsp cold water to bring the dough to a ball. Turn the pastry out and wrap it in clingfilm. Chill for half an hour. Preheat the oven to 200°C, fan 180°C, gas 6.
Roll out the pastry and use it to line a 20 x 30cm loose-based tin. Put the tin on a baking tray, line with baking paper and baking beans and blind bake for 20 minutes. Remove the beans, reduce the oven temperature to 180°C, fan 160°C, gas 4 and bake for another 10 minutes. Trim off the excess pastry and leave to cool.
To make the filling, mix the cornflour, sugar, lemon zest, lemon juice and 300ml water in a pan. Bring to a simmer, stirring constantly, and bubble for a couple of minutes until thickened and smooth. Remove from the heat and beat in the butter until melted. Beat the egg yolks, stir into the pan and return to a low-medium heat. Keep stirring for a few minutes, until the mixture thickens, then tip the filling into a bowl to cool.
Blend the blackberries and icing sugar to a purée in a food processor, then pass through a sieve. Brush the tart base with some of the blackberry purée. Pour the lemon filling into the tart case. Chill for about an hour.
To make the meringue, put the sugar in a pan with 75ml water. Heat gently until the sugar has dissolved, then boil for 4-5 minutes or until the temperature reaches 120°C on a sugar thermometer. Whisk the egg whites in a large bowl to firm peaks, then slowly pour in the syrup in a thin stream, whisking all the time. Keep whisking for a few minutes until the meringue has cooled. Spoon the meringue into a piping bag with a star nozzle. Pipe little blobs then pull upwards to leave a peak on each one.
Brûlée the tips of the meringue with a blowtorch to give an impressive finish.