"Just desserts" is the bastardized, ignorant version of the proper version.
de·sert2
Pronunciation Key (d-zûrt)
1. Something that is deserved or merited, especially a punishment. Often used in the plural: They got their just deserts when the scheme was finally uncovered.
2. The state or fact of deserving reward or punishment.
see deserts
See post 63. I don't know everything just like you don't know everything. However, it took two online dictionaries before I found the definition myself. Have a good day.