In a stand mixer, with paddle attachment, cream the butter thoroughly.
Beat in the sugar until light and fluffy.
Whisk the salt into the flour, and gradually add to the butter mixture, gently mixing well.
Knead the dough very gently until smooth and soft but not oily.
Pat dough into an ungreased 38 cm x 26 cm baking sheet. (See Recipe Note 1 for variations)
Smooth the surface and score into 6 cm x 2.5 cm rectangles.
Prick the top of each rectangle 3 times with a fork.
Cover lightly with cling film and place in the refrigerator for 2 hours.
Preheat the oven to 160 ℃
Cut out the rectangles and place 2.5 cm apart on ungreased baking sheets.
Place in the oven and immediately reduce the temperature to 140 ℃
Bake for 30 minutes, then turn the sheets around bake an additional 15 minutes until firm, golden on the bottom and sand coloured on the the top.
Allow to cool for 10 minutes on the baking sheet before removing to wire racks to cool completely.