The other day one of my sons asked us “After saffron what is the most expensive spice?” I answered “if it’s considered a spice then vanilla”.
Which, of course, was the answer… I’ve never thought of vanilla as a spice.
BTW, I cannot find the original item where Baker Aunt talked about vanilla in shortbread but I have vanilla in mine. I checked Jamie Oliver’s “Cooking with Jamie” which is the closest thing I have to a Scottish cookbook (I bought it at a book store in Scotland) and Mr. Oliver DOES NOT USE vanilla. So maybe I will take it out of mine.
When I went to pastry school at SFBI, our instructor said that the most expensive ingredient we used all week was pistachio paste, even more expensive than the gold foil we decorated some pastries with.