Your just deserts are the consequences you justly deserve. It’s a single S. Sometimes people incorrectly write “just desserts”.
It’s a bit confusing because it’s pronounced like the sweet course at the end of a meal but spelled like the word meaning a waterless, desolate area of land with little or no vegetation, typically covered with sand.
Just remember: it’s related to deserve – single S.
You can read about the correct spelling of just deserts and a whole lot more, in the Clifford & Co editorial style guide.