The sentence that is written correctly is:
Snow cones, a tasty treat, can be found in many delicious flavors.
This sentence is correct because the commas that introduce the phrase a tasty treat do not interrupt the grammatical structure as in the other sentences. In this case, the commas introduce this phrase as a postmodifier to the subject snow cones without interrupting the structure subject + predicate that is usually followed.
Meanwhile, in the rest of the sentences, there is a comma that either separates the subject from the predicate (incorrect) or that is just unnecessary, like in the first sentence.
