In baking, opposites usually attract. Sweet and salty. Cold and hot. And in the case of these cookies, mellow, sweet shortbread and tart, acidic key limes. You could use limes for this recipe, but the cookies won't really be the same. Key limes have a unique, tart flavor, with subtle yet discernible floral notes. I love limes, but to be honest, they taste a bit one-note in comparison.
Using key limes does mean that these cookies will take a little more work to prepare, given key limes' thin skin, higher seed density, and small size. In short, zesting these guys is a pain, but totally worth it.
Luckily the base cookie, which is essentially a French sable, is easy to prepare with ingredients you probably already have in your pantry (sugar, flour, butter, eggs, vanilla). The dough is one of my favorites to make ahead. Roll it into a log, cover it in plastic wrap, stick it in the freezer, then slice and bake when you're in the mood for some freshly baked cookies. Which, in this weather, is every day at 4 p.m.