Learn Spanish (Spain): ¿Y de postre? Un helado de chocolate. - And for dessert? A chocolate ice cream.

In Spanish, when you talk about what you are having for a specific part of a meal, you use the preposition de. So de postre means "for dessert", just like de primer plato means "for the first course".

Why is it helado de chocolate and not chocolate helado?

In English, "chocolate" is put before the noun to describe the flavor. In Spanish, you state the main item first (helado), add de (of), and then the flavor or ingredient (chocolate). It literally means "ice cream of chocolate".

Can I just ask ¿Y postre? without the word de?

You really need the de here. If you just ask ¿Y postre?, it sounds like you are asking if dessert simply exists or is included in a menu. Asking ¿Y de postre? specifically asks what someone is having for the dessert course.

How is Y being used at the start of this question?

While Y literally means "And", starting a question with it is a common way to say "What about...?" in casual conversation. Here, it shows the speaker is smoothly moving on to the next part of the meal order.