Preheat the oven to 375 degrees Fahrenheit or follow instructions of package of puff pastry.
Cut the puff pastry into 8 equal-sized pieces (keep them rectangular). Put a teaspoon full of jam in the middle of each pastry rectangle. Fold up two opposite corners of the pastry and join in the middle. You'll also need to curl up the other two corners of the pastry because otherwise when the pastry puffs up the jam will get pushed off onto the baking tray.
Place on a greased baking tray and brush the top of each pastry with the whisked egg. Bake for approximately 10-15 minutes or until the pastry is puffed and golden.
Cool for a few
Sift the flour and salt into a large mixing bowl, then put the bowl in the fridge for a few minutes to chill. (Keeping the flour and bowl cold will help you to get a better result later and create nice layers of pastry.)
Meanwhile, cut the butter into small cubes.
Using a round-bladed knife, stir the butter into the bowl until each piece is well coated with flour.
Pour in the water, then, working quickly, use the knife to bring everything together to a rough dough.
Gather the dough in the bowl using one hand, then turn it onto the work surface. Squash the dough into a fat, flat sausage, without kneading. Wrap in cling film then chill the dough in the fridge for 15 minutes.
Lightly flour the work surface and the pastry. Roll out the pastry in one direction until it’s about 1cm thick and three times as long as it is wide, or about 45x15cm/18x6in. Straighten up the sides with your hands now and again, and try to keep the top and bottom edges as square as possible.
Fold the bottom third of the pastry up, then the top third down, to make a block about 15x15cm/6x6in. It doesn’t matter if the pastry isn’t exactly the right size - the important thing is that the corners are nice and square.
Turn the dough so that its open edge is facing to the right, like a book. Press the edges of the pastry together with the rolling pin.
Roll out and fold the pastry again, repeating this four times in all to make a smooth dough, with buttery streaks here and there. If the pastry feels greasy at any point, or starts to spring back when as you roll, then cover and chill it for 10 minutes before continuing.
Chill the finished pastry for an hour, or ideally overnight, before using.
When you use the pastry roll it out to about the thickness of a £1 coin, and cook it at 200C/400F/Gas 6 for about 30 minutes.