Preheat oven to 350°F.
Cream the butter and sugar in a stand mixer with the whisk attachment, or using a handheld mixer. Beat for 2 minutes until light and fluffy.
Sieve together the salt and flour.
Add flour to butter mixture in 4 batches. Mix until the flour is just incorporated each time. The dough will look like wet sand but will hold together when squeezed into a ball.
Gather up the dough from the bowl and push it together with your hands to create a ball. Put out on to a clean work surface.
Knead the dough briefly to bring it all together. It will stick together but a few crumbly bits may escape here and there.
Divide dough into 4 equal pieces and place 2 balls each into two 9-inch x 9 -inch pans.
Using your hands, push the dough into the pan. Use a ruler to ensure that the dough remains less than ½ inch thick.
Place a piece of parchment paper over the cookie dough. Use the back of a measuring cup to smooth out the top of the cookie dough by pressing on the dough over the waxed paper. Repeat with the other pan.
Use a sharp knife to score the dough into rectangles. Only score the top, don't cut the whole way through. (I usually do 3 columns down and 5 or 6 across - but make them bigger or smaller as you wish)
Bake for 30 minutes or until the cookies are set and the edges are just turning golden.
Remove from the oven to a cooling rack. Use the same knife to score the cookies again following the same lines, making sure to cut all the way to the bottom.
Allow the cookies to cool in the pan for 20 minutes before removing and allowing further cool on a wire rack.