Caster Sugar - Caster sugar is finer than granulated sugar, but not as fine as powdered sugar. To make your own, simply run granulated sugar through a coffee grinder for 20 seconds
Butter - Use good quality butter. Since there is no vanilla in this recipe, the butter carries the flavor for the most part.
Proportions - This recipe uses the traditional Scottish method of 1 part sugar - 2 parts butter - 3 parts flour by weight. Cool, eh?
You can also roll the cookie dough into a log, chill, and slice into rounds. If you go down this route, then reduce the baking time. Check the cookies after 15 minutes and then continue checking every 5 minutes until they are done. They should be done in less than 25 minutes. The baking time will depend on the thickness of the cookies.
Shortbread cookies develop more flavor as they sit in the jar. If you are serving this to guests, I recommend making them a day or two ahead.