if mariposa is out, then you can find some great deserts at catcha (i know you said beach but hey nobody seems to read what people write anyway), sunrise club (also some great breakfast crepes which are also available as desert and the best espresso and cappucino- i usually make it a point to get an affogato- chocolate gelato with an espresso shot poured over it) and as caribsoul mentions kuyaba which i think also got mentioned before.
also pushcart and rockhouse have some good desert items, some puddings and cakes.
you can also pop into most of the liTTle shops and such and scoop up tons of sweets and pastries.
i'm sure plenty of places have a little undiscovered desert chef on staff doing their thing, negril inn had one of the best back in the day.