The next day, take the butter sheet and roll down to a 17 cm x 24 cm rectangle, 6 mm thick. Roll the dough to twice the size of the butter (34 cm x 24 cm). Place the butter in the center of the dough, fold the dough back to the middle and give two single turns. Leave to rest for one hour. Meanwhile, soften the salted butter. Remove the dough from the fridge and roll down to prepare for another single turn. Before doing the turn, use a palette knife to spread the soft salted butter 2/3rds over the length of the dough. Cover with the granulated sugar and demerara sugar, and fold to finish the single turn. Leave the dough to rest for one hour in the fridge. Widen the dough to 27 cm in width and roll down to 55 cm in length. Place onto a tray, cover in cling film and place into the freezer for 10 min to firm up. Line the tins* with the soft butter, then coat with the mixed sugars and salt and store in the fridge. Roll the dough lengthways to a thickness of 3.5 mm and cut into 9 cm x 9 cm squares. Fold each square corner back to the center and press gently to hold the points in place. Place each folded square face down into the lined tins with sugar.
*Equipment: 24 aluminum foil tins, base size 7.5 cm diameter x 3 cm deep