The trick to satisfying sandwich cookies is to make them delicate. Slathering melted chocolate between two already-rich-enough chocolate chip cookies is not my idea of a successful sandwich cookie.
First of all, the cookies need to be light—thin, flaky, crisp, or lacy. Such bases can support any kind of filling. Generally, rich cookies are sandwiched easily and successfully with reduced jam—who can resist a little round window of raspberry glaze peeking out between two buttery cookies? Sometimes if the cookies are both rich and buttery they may be sandwiched with chocolate—a little excessive, but that’s part of the thrill.