The Cantonese term "dim sum", meaning "dot hearts," or "small snacks," is plural. Some speakers of English, used to hearing and seeing the "s" plural ending on nouns, are now perceiving "dim sum" as a singular noun, as you have noticed, and saying "dim sums" for the new plural. This practice is not established, however, and "dim sum" is the standard form--for now, anyway.
If you want to use the term for the individual dishes that make up dim sum, you can say either
ha gau or siu mai
This rather arcane information is courtesy of
Hongkong Online Style Tips, (HOST, the on-line version of The Young Reporter Stylebook) athttp://www.hkbu.edu.hk/~jour/style/host_intr.html
The process of altering foreign words to fit native speech patterns is universal in human language, and nothing can be done to stop it. Remember that the Latin word "data" is now used as a mass noun ('this data is flawed"), although it is the plural of the Latin singular "datum" ("item of information").
Similarly, the Greek plural "criteria" is being heard more and more as a singular noun instead of "criterion," the Greek singular. These deviations from the norms of the mother language may set our teeth on edge, but to protest is usually hopeless.
About thirty years ago I was bemoaning the use of "data" as a mass noun to an elderly professor of classics. He replied quietly, "I'm still mad about "agenda." "Agenda," of course, is the plural of Latin "agendum," "thing to do." It's now a full-fledged singular noun in English, and few English speakers remember it any other way.