This cake smells good.

This cake smells not smelling is appropriate likewise some other verbs as understand, help, etc. instance: I understand is appropriate not, I’m understanding. If you need some more clearance, read English grammar's verb transitive and verb intransitive section.

One note, if you leave the term simply as "smells" with no description of how it smells, people will think it has an unpleasant smell. The term 'smell' carries a negative connotation and you need to pair it with a nicer word to describe something that smells good.

THIS CAKE SMELLS WONDERFULL. YOU SHOULDNT USE ING.