Use desert when you are referring to the arid landscape of endless sand where the camels roam and water is precious. It is also the correct term when you say "just deserts". This latter meaning arises from its origins in the word "deserve/deserving". Perhaps it is commonly misspelled as "just desserts" because everyone thinks that desserts are definitely something worth deserving... Avoid thinking this way and always think "deserving", which has but one "s"!
Use dessert when you are referring to those gooey, delicious, sweet treats that follow a main course. Yes, after eating all your Brussels sprouts up, your desserts are just deserts!
- Consider keeping a notebook of difficult grammar issues to revise frequently.