Beat butter and sugar in a medium bowl with an electric mixeruntil pale and fluffy. Stir in 1 tablespoon water and sifted flours, in two batches, until combined.
Turn dough onto a lightly floured surface; knead gently until smooth. Divide dough in half; enclose each half in plastic wrap. Refrigerate for 30 minutes.
Preheat oven to 160°C/325°F. Grease two large oven trays.
Roll one dough half between two sheets of baking paper until 1cm thick; freeze for 15 minutes. Peel away top layer of baking paper; cut 4cm x 7cm rectangles from dough. Slide baking paper and rectangles onto one oven tray. Mark down the centre of each rectangle with the back of a knife into two fingers; using a fork rough the surface of one finger. Freeze for 15 minutes or until firm. Re-roll scraps and repeat with remaining dough until all the dough has been used.
Bake for 25 minutes. Remove from oven. Working quickly, use a palette knife to straighten the edges of each biscuit. Return to oven; bake for a further 15 minutes or until a biscuit can be pushed gently without breaking and are browned lightly. Leave on trays for 5 minutes before transferring to wire racks to cool.