Learn Spanish (Latin America): ¡Qué sabroso es el maíz tostado!

It is a past participle acting as an adjective. It comes from the verb tostar (to roast or toast). To form the regular past participle of an -ar verb, you drop the -ar and add -ado, giving us tostado (roasted).

Does tostado have to change its ending depending on the noun?

Yes! When a past participle is used as an adjective, it must match the gender and number of the noun it describes. Since el maíz (the corn) is a masculine, singular noun, we use the masculine, singular ending -o for tostado.

Why do we use Qué to mean "how" in this sentence? Doesn't cómo mean "how"?

While cómo usually means "how" for asking questions, when making an exclamation about a quality (like saying "How tasty!" or "How beautiful!"), Spanish uses Qué. So, ¡Qué sabroso! translates to "How tasty!".

Why is the verb es placed right after sabroso instead of at the end?

In Spanish exclamations that start with Qué plus an adjective, it is standard to put the verb before the subject. So the word order in this sentence is literally "How tasty is the roasted corn!".