Sift the flour and salt together into a bowl. Make a well in the middle, add the lemon juice, and stir with a knife, adding enough cold water to make an elastic dough.
Place on a floured board and knead for five minutes until the dough is smooth and doesn't stick to your fingers.
Roll the dough into a rectangle large enough to cover the butter. Put the butter in the middle of the dough and fold the dough ends over the butter.
Flatten the dough with a rolling pin, then roll out gently (so the butter does not come through the dough) into a long strip.
Fold the ends of the dough towards the middle, forming three layers. Press down gently. Let pastry rest in a cool place for 15 minutes.
Place the pastry on a floured surface with the unsealed edge towards you. Roll and fold as before. Turn and repeat three more times.
Leave the pastry to rest in a cool place for 10 minutes.
Place the pastry on a floured surface with the unsealed edge towards you as before. Roll and fold again, turning and repeating three more times, as before.
The pastry is now ready for use. Use in your favorite recipe.