Sean has asked for this recipe and some commentary on technique, so here it is, Sean, just for you.
Scottish shortbread requires only a few ingredients ... the basics of all shortbread (butter, flour and sugar), then adds in a little egg and vanilla and a hefty two pinches of baking soda. Here is the recipe:
1) Keep the work surfaces and everything cool
2) Work quickly so no gluten has a chance to develop. The 'short' in 'shortbread' means no gluten development.
3) Do full mis-en-place
4) Pre-heat the oven for 30 minutes before baking, and position the oven racks before turning on the oven! One of the secrets to good baking is clean temperature management ... once the oven is hot, don't have the door open for more than 2 seconds at any time (to reduce temperature fluctuation).
5) If using parchment paper on a baking sheet, make sure the paper only covers the bottom of the sheet, with no 'ears' sticking up to reduce heat movement.
6) Remember to cool your shortbread on a rack for at least 10 minutes before service, and cool to room temperature before packing or boxing for keeping or shipping.
** Final Note ** Remember to use a cutter with no sharp spiky shape-parts ... gently rounded, circular, oval or slightly-bulging rectangles work best. Do not use a wash on top of any sort. Prick for traditional look (see photo).