气 means steam, air, vitality (of a person), aura etc. So in this case, the heat wave/steam emanating from the bun.

I soppose "qi" means 汽 steam, vapor. Homophones are common in Chinese. A syllable in a particular tone may stand for quite a few frequently used morphemes/words/characters, and a syllable in all four tones may correspond to up to tens of them. Therefor homonymy is a challenge for CFL learners.