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First, make the pastry. Tip both flours and the sugar into a bowl with a large pinch of salt. Use your fingers to rub the butter into the flour mixture until it resembles crumble topping, then rub in the soft cheese until the pastry comes together. (If it’s still crumbly, add 1 tbsp iced water and it should come together.) Tip onto a surface and knead for 30 seconds, then cut off about a third of the pastry. Mould both pieces into flat rounds. Wrap and chill for at least 1 hr. Will keep chilled for up to two days.
Wash and drain the blackberries, being careful not to break them up, then tip into a bowl with the sugar. (If they are wild blackberries, they will need slightly more sugar.) Toss together and set aside while you roll out the pastry.
Butter a 23cm pie dish and, on a lightly floured surface, roll the larger piece of pastry out into a round that fits the base of the dish with some overhanging. Push the pastry down in and around the dish to line it. Sprinkle the cornflour over the blackberries and gently mix together. Tip the blackberries and all their sugary juices into the pastry case and dot with the butter. Beat together the egg and milk to make a glaze, then brush the edge of the pie with some of it.
For an easy pie, simply roll out the rest of the pastry, drape it over the top, then trim and fork or crimp the edges together. For a lattice finish, roll the remaining pastry into a square, roughly 30 x 30cm. Cut out 10 strips of roughly the same width. Lay five of the strips vertically over the pie, then weave with the remaining five horizontally, to create an even lattice over the fruit. Or, if you prefer, you can cut each strip into three thinner strips to weave a more delicate lattice. Pinch the edges, then trim the excess and crimp. Can be assembled and chilled four hours before baking, or baked straightaway.
Heat the oven to 200C/180C fan/gas 6 with a baking tray inside. Brush the top of the pie with more egg glaze and bake for 20 mins, then brush with the glaze again, scatter generously with extra sugar and bake for another 20-25 mins until the pastry is a deep golden colour and the juices are bubbling. Leave the pie to cool for at least 30 mins, then serve as is, or with ice cream.