Gran's Scottish Shortbread

Beat butter with sugar, salt, and egg yolk until fluffy.

Add flour a little at a time until mixture is too stiff to work with a spoon. (Do not use a mixer, as the dough will be far too stiff.)

Flour a counter or board and knead the dough, pulling the flour into it, until it just begins to "crack". It's about the texture of play-doh at this point, perhaps a little stiffer. You may need a little more or a little less than 2 1/4 cups.

Divide the dough into two balls and pat them each into a circle about 1/2 inch thick on an ungreased cookie sheet. Prick the whole thing with a fork, and use the flat of the fork to press lines around the edges.

Bake at 350°F until brown, about 30 minutes. The center will be a pale pastel yellow and the edges will just be turning brown.

Remove the cookie sheet and cut the shortbread while it's still hot (it gets more brittle as it cools).