Line an ungreased 8-inch square pan with parchment pape.
Soften the butter and stir in the sugar, salt, and egg yolk.
Add flour, a little at a time until mixed in. The dough will look a little crumbly.
Knead for a short time until dough can form a ball. There will be some cracks—it won’t be 100% smooth. Don’t over-work the dough.
Pat dough out in the prepared pan and chill in the fridge for 30 minutes.
When ready to bake, preheat oven to 325 degrees Fahrenheit.
Remove dough from the refrigerator. Use a sharp knife to cut into slices. Use a dinner fork to make holes in the dough.
Bake for about 20 minutes until lightly browned.
Notes
Nutrition information is an estimate.
Sugar
Mom’s recipe calls for extra-fine sugar. Extra-fine sugar is also known as caster sugar, baker's sugar, or super-fine sugar. I splurged and bought the super fine sugar, it cost about $5 for a one-pound bag.I have seen recipes for shortbread that use powdered sugar. I used that in one batch and it did fine. But for overall flavor/texture I give a slight edge to the super-fine sugar.If you use regular sugar, the recommendation I’ve seen is to run it through your blender first.
Butter
The recipe calls for unsalted butter. I recommend sticking to unsalted so that the amount of salt is correct. When a recipe only has five ingredients, you want to use a good quality butter because that is going to be the primary flavor for this cookie.
Flour
The recipe calls for all-purpose flour. As with the butter, use a good quality all-purpose flour. One thing I look for is for it to be unbleached. Choosing organic is great too. Most supermarkets carry a good selection of flours these days.
Mixing the Dough
My mom’s recipe has all the mixing done by hand, so you don’t actually need a mixer. I did one batch completely with a mixer, a second batch where I mixed the butter and sugar with the mixer and finished by hand.