A lamentably common error. The phrase is properly spelled "just deserts."
Ordinarily I wouldn't go grammar nazi on this, but given the seriousness of the thread...
Using just desserts is not a serious error, and it is much more common than just deserts in 21st-century texts. Some people still consider it wrong, however. Whether to pay this any heed is for each of us to decide for ourselves.
http://grammarist.com/spelling/just-deserts-just-desserts/
Works for me. Sit on it, Buttons!
"The expression meaning that which is deserved was originally just deserts. The phrase is the last refuge of an obsolete meaning of desertnamely, something that is deserved or merited. But because most modern English speakers are unfamiliar with that old sense of desert, the phrase is often understandably written just desserts.
Using just desserts is not a serious error, and it is much more common than just deserts in 21st-century texts.
Some people still consider it wrong, however. Whether to pay this any heed is for each of us to decide for ourselves."
Thanks for pointing out to me and everyone else that you are not up to snuff in your use of the modern 21st-century English language. :-)