Place the flour, sugar, salt and yeast into the bowl of a stand mixer fitted with a dough hook. Add the egg, water and milk. Mix on a slow speed until combined, then increase the speed and mix for 5 minutes to form a dough.
Tip the dough onto a lightly floured worktop and shape into a disc. Wrap in cling film and chill for 1 hour, then unwrap and roll it out into a 1cm-thick rectangle, measuring 45 x 20cm.
Flatten the butter into a 30 x 18cm rectangle, then lay it on the dough so it covers the bottom 2/3.
Fold down the dough to cover 1/3 of the butter. Cut off the exposed butter, without cutting into the dough. Place this butter on top of the folded dough and fold the bottom 1/2 of the dough up. Squeeze the dough edges to seal. Wrap in cling film and chill for at least 1 hour to firm up.
Place the dough, short side facing you, on a lightly floured worktop. Roll into a 45 x 20cm rectangle. Fold 1/3 of the dough up and the top 1/3 down. Chill for 1 hour. Repeat twice more, chilling between folds. After the final fold, chill overnight.
Make the crème pâtissière. Heat the milk until it just boils. In a bowl, beat the yolks with the sugar, vanilla paste and cornflour, then pour in the hot milk, and whisk. Transfer to the milk pan and place over a low heat. Whisk until the mixture just begins to boil and thicken. Transfer to a bowl, fold in the lemon curd, then cover the surface with cling film.
Roll out the pastry to 7mm thick and cut it into 18 squares of 8cm. Make diagonal cuts 2.5cm from the corners of each square. Place the squares onto the baking sheets. Gather every second point of dough to the centre of each square, and press down to make a hollow. Cover the sheets loosely with cling film and leave to prove for 2 hours, or until doubled in size.
Heat the oven to 200°C/180°C fan/400°F/ Gas 6. Put 1 teaspoon of lemon crème pât into the hollow of each pastry. Brush the dough with beaten egg, then bake for 15–20 minutes, until golden.
While still warm, brush the pastries with the warmed apricot jam. Transfer to a wire rack to cool. Drizzle the cooled pastries with lemony icing sugar to finish.
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