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Heat oven to 180C/fan 160C/gas 4. Line two baking sheets with parchment. Place the flour, baking powder, butter and ½ tsp of the flaky salt in a food processor, then whizz for a minute until the butter is completely mixed with the flour. Add 4 tbsp water and pulse until the dough comes together. If it still feels dry, add 1 tsp more water and process until you have a soft but not sticky dough.
Roll out the dough on a lightly floured surface into a rectangle approximately 50 x 25cm and as thin as possible. Brush a little water over the surface of the dough, scatter 1 tsp salt flakes over and press in lightly. Prick the dough all over with a fork, then cut into 18 squares. Place on the prepared trays – don’t worry if they stretch a bit. Bake for 10-15 mins until the biscuits feel dry and sandy but are still pale – they may still feel soft but will harden up when cooling. Transfer to a wire rack and leave until completely cool. Will keep in an airtight container for up to 2 weeks.