I am unsure why this spelling error is so difficult, but it is one of the most common that I have seen.
I wish I had a quick cheat to remember which one is which (Maybe you do?) but I really don’t.
Desert is a noun, and a verb. You can desert someone (verb), or you can leave a person in the desert (noun).
Dessert is just something sweet that we all wish we could eat in excess and never pay the consequences.