I've been wondering, Is there a tag for steamed buns like the one Letty is eating?

nikuman perhaps?

tag autocomplete finished meatb to meatball instead of meatbun (which is aliased to nikuman)

That bun is either manjuu or nikuman.

Unfortunately, it will be quite hard to differentiate between the two.

Manjuu is often smaller and may have a harder bun. Also, they're almost always be filled with sweets.
Nikuman or meatbuns are often quite large, the bun is always soft, and most of the time it's filled with meat, though there's also some that is filled with chocolate or bean paste.

That bun is definitely a nikuman.

Well, I learned something today. Thank you.