Preheat the oven to 350 degrees F.Lightly grease two baking sheets.Beat together the butter and powdered sugar in a large bowl until combined.(Creaming until light and fluffy is not necessary for shortbread.)Add the flour and salt and work together to form a stiff dough, then knead lightly for a few moments until smooth, but avoid over handling.Roll out the dough to a thickness of about ½ inch.Cut into 24 cookies (re-rolling the trimmings) and place onto the baking sheets, well spaced to allow for expansion.Bake for 12-15 minutes until pale golden brown.Cool the cookies for a few minutes, then transfer to wire racks to cool completely.When cold, brush the edges with lightly beaten egg white and dip into the Demerara sugar.
Source : The Best of Billington’s Recipe Collection 2