Spray a 9 x 13 inch baking pan with nonstick cooking spray.
Cream together softened butter and sugar in a large mixing bowl.
Add flour and salt and mix until blended.
Press into the bottom of prepared pan and bake for 15 minutes.
While crust is cooking, make filling by mixing together sugar, eggs, lemon zest, lemon juice, and flour. Pour filling over hot crust and bake for about 25 -30 minutes or until set.
Remove from oven and dust with powdered sugar.
Cool completely and cut into squares.
Notes
If you can gather up enough self control to not devour the whole pan, you can store them for 2-3 days in the refrigerator.