The best way to slice shortbread is while it's still warm. I let it cool for about 10 minutes in the tart pan, then lift it out of the pan on the removable bottom. From there I usually gently remove the tart from the metal bottom plate. This can be tricky, and I find a very thin metal cake lifter or spatula works great. (You can slice the tart right on the metal plate if you like.) I slice the shortbread using a large sharp knife. Sometimes it helps to make shallow score marks where you're going to cut, but generally it is soft enough to slice easily when it's warm.