Cream butter in a medium mixing bowl; gradually add sugar, beating until light and fluffy. Add flour and salt; stir until well blended. Divide dough into three equal portions; chill at least 1 hour.
Press one portion of dough into a 7- x 5-inch rectangle on a lightly greased cookie sheet; press with tines of a fork around outer edge of rectangle. Divide rectangle into 2 1/2- x 1-inch bars by making deep indentions with a wooden pick. Repeat procedure with remaining portions of dough.
Bake at 300° for 30 minutes; cool slightly. Break warm shortbread into bars. Remove from cookie sheets, and cool completely on wire racks.
Note: Chilled dough may be pressed into a floured shortbread mold and turned out onto a lightly greased cookie sheet. Bake as directed.