Well it most recently used to be cookie on b'fast/lunch flights > 4 hours and ice cream sundae on dinner flights > 4 hours. The "desert" served alongside lunch on these flights was usually that caramel-nut-chocolate concoction.

As discussed in another thread, they're taking sundaes off many (most/all?) of the longer non-transcons and they took out the cookie/sundae detail from the website, so anything goes now I guess...