This the word for word description of how to make my Scottish friend's Grandmothers recipe. The cookies are wonderful and my favorite after years of trying other recipes.
She has modernized it by doing in the food processor. I've left in all of her comments...in her words. It turns the recipe into a small visit with her, and that is great because she has now moved back to Scotland and I miss her so.
Cream the butter and sugar; if you use a food processor, taking the butter out of the fridge doesn't matter-it just takes a little longer to cream.
Work in the flour. Again, if using a food processor, bung in about 11 oz of the flour, work that in and then bung in the rest.
Plop the whole blob (even if some is still not worked in) onto your work surface and finish the working in by kneading until it just starts to stick a little. (Sometimes people, like my ancient and now deceased cousin Isabel, woffle on about not handling it very long as it will make the shortbread tough. Rubbish I say. Carry on and knead all you want. It's fun and therapeutic if you're a bit irritated about something)
Put the lump of dough into your greased-with-the-butter-paper pan, and squish it over the pan with your fingers. Then smooth it out. Merle found out a spoon works the best after I'd been using a knife for years, just because that's how my mother used to do it. Then, with a fork (no Merle refinements on that one) prick it all over and make any twinky little patterns you like.
As far as temperature goes, the recipe of my grandmother works fine. Bake at 350 for 1/2 hour; turn down to 275 and give another 1/2 to 3/4 hour until golden brown. The thing is to leave it in there long enough to cook it without burning it (guilty on several occasions).
Cut when warm, or you're in trouble. Cool before taking out of the pan, or you're in trouble again.
NOTE: To approximate the "caster sugar" (which is a very fine granule sugar) put into food processor, and process on high for about 1 minute.