Preheat oven to 300 F.
Sift flour and salt, mix into bowl and set aside.
Mix butter on high speed until fluffy (3 to 5 minutes).
Gradually add sugar slowly, continuing to mix until pale and fluffy.
Add flour all at once and mix until combined.
Butter a square pan.
Pat and roll shortbread into pan no more than ½-inch thick.
Refrigerate for at least 30 minutes.
Cut into squares.
Bake until golden and make sure the middle is firm; approximate bake time 45 to 60 minutes.
Cool completely. Re-slice them, if necessary, and serve.
