I think I'd be inclined to translate 饅頭 as "bread" rather than "bun"... not entirely sure why that feels more right to me, given that they're more similar in size to a British bun than a British loaf of bread, but I'm pretty sure I've seen frozen packaged 饅頭 with "Chinese steamed bread" on the package. Perhaps it's because buns are round, and 饅頭 are often more sort of cylindrical? (I'm not sure how to describe that shape, but I expect you know what I mean!)