Popdragons are mentioned once in How to Speak Dragonese as part of a meal served to the Fat Consul.

"He was eating a light snack of double reptoburger with tomato-and-dormouse relish and popdragons on the side."
How to Speak Dragonese

Popdragons may be a distinct species of dragon, or perhaps it simply refers to dragons prepared in a certain way, like popcorn. However, if the latter is the case, it is extremely difficult to roast whole dragons as their skins are, at a minimum, fairly fire resistant.