Answer:
Round as an apple, deep as a cup, and all the kings' horses can't fill it up. What is it?
Always wax, yet always wane: I melt, succumbed to the flame. Lighting darkness, with fate unblest, I soon devolve to shapeless mess.
I cost no money to use, or conscious effort to take part of. And as far as you can see, there is nothing to me. But without me, you are dead.
I can be written, I can be spoken, I can be exposed, I can be broken.
It holds no blessings in disguise. Its rhymes are aimed at your demise, it's cast only to ruin, Whatever you are doin'.
This food is a tortilla filled with cheese and sometimes chicken before being heated or cooked.