Cut the butter into cubes and leave at room temperature to soften.
Fit the K beater to the machine.
Line the baking tray with parchment paper.
Add Ingredients 1 (butter, flour, salt, sugar) into the Mixer bowl.
Attach the Mixer bowl to the machine and fit the splash guard.
Mix on speed 2 for 2 minutes.
Add Ingredients 2 (egg, milk) into the Mixer bowl.
Mix on speed 2 for 1 minute.
Place the pastry on a lightly floured worksurface and knead together.
Form a flat circle and wrap in cling film.
Chill in the fridge for about 30 minutes.
Pre heat the oven to 190ºC.
Transfer the pastry from the fridge to a lightly floured worksurface.
Roll out the pastry into a large circle about 1 mm thick.
Carefully transfer the rolled pastry onto the baking tray.
Add Ingredients 3 (blackberries) to the pastry.
Leave a 6 cm border around the edge.
Sprinkle Ingredients 4 (sugar, lemon zest) over the dough.
Fold the pastry edges up around the blackberries, leaving the centre exposed.
Bake for 10 minutes at 190ºC.
Brush the pastry with the first item of Ingredients 5 (egg).
Sprinkle the pastry with the last item of Ingredients 5 (caster sugar).
Bake for 25 minutes at 190ºC.