Looks to me like a bantam dark Brahma pullet. Does she have a pea comb?
If she has a single comb she could be a Birchen Cochin bantam. They are very similar breeds.
Are her legs yellow under the feathering? Both breeds have yellow shanks.
Yeah, looking at the tail and general roundedness, She does look more like a Cochin, and the little wattles probably mean that she has not got a pea comb like a brahma would, because then the wattles would grow in more slowly. Birchen Bantam Cochin pullet, then, most likely.