Questions & Answers about Qual é o seu prato favorito para o jantar?
In this question, qual is closer to which in English: you’re asking which (one) is your favorite dish among all possible dishes.
Portuguese often uses qual é before a noun like this: Qual é o seu prato favorito?
Using Que é o seu prato favorito? is not natural here; que is more common before verbs (e.g. Que você quer?) or as o que.
Both are correct.
Qual é o seu prato favorito para o jantar? is the full form and is completely standard.
In everyday Brazilian Portuguese, people often drop é and say Qual o seu prato favorito para o jantar?; this is very common in speech and also appears in writing.