nuances - 朝ご飯は vs. 朝ご飯に - difference between these two sentences?

I wanted to ask my language partner what he ate for breakfast. However, I was unsure whether what I said was correct despite him telling me that it's good - I would just like some verification.

I said:

朝ご飯は何を食べましたか。

I'm unsure whether this is "correct" as in is this how one would normally ask another person what they ate for breakfast?

The Google-translated equivalent is:

(あなたは)朝食に何を食べたのですか？

I don't like to rely heavily on Google-translated sentences to verify if what I mean/say is correct. What is the difference (if any) between these two sentences and is what I said grammatically correct?

Answer

朝ご飯は何を食べましたか。

This is correct. You're literally saying, "As for breakfast, what did you eat?"

朝食に何を食べたのですか？

Google is using the particle に to indicate at in a similar manner to that of 今週末に (this weekend), however, に is usually omitted for relative times. They're both grammatically correct. "At breakfast, what did you eat?"