In a large bowl, cream butter, sugar and salt until pale and fluffy.
Knead flour into creamed mixture to make a workable dough.
Press into a ball and wrap in foil or plastic wrap.
Refrigerate 30 minutes.
Preheat oven to 375°F (190°C).
Grease a baking sheet.
On a lightly floured surface, roll out dough 3/4 inch (1.5 cm) thick.
Trim to fit baking sheet.
Place on greased baking sheet.
Pierce dough several times with a fork.
Bake 25 to 30 minutes or until golden.
Cut warm shortbread into fingers with a sharp knife.
Sprinkle with additional sugar.
Cool on baking sheet until firm then cool completely on a rack.