This basic, buttery shortbread practically begs that you customize it to suit your own tastes or pantry supplies. The dough, which comes together quickly in a food processor, is already enhanced with rosemary, but nuts, seeds, citrus zest, spices, vanilla or minced dried fruit — or a combination of some of these — all make fine additions. Scale it up, scale it down. Add more salt, or use less. As long as you maintain the butter-flour ratio (one stick of butter for every cup of flour), you…