manju

A Japanese confection/sweet, they are made out of flour, rice powder, buckwheat and are usually filled with anko (red bean paste)
Not to be confused with Mantou, a similar type of bun that's native to China.

Their relation to Yukkuri came about after Makako conceptualized Yukkuri as living Manjus, then (mostly abuse-oriented) artists started using bean paste like the Yukkuri's equivalent to blood for whenever they got hurt or died, so it became widely accepted that they were living Manju, that have different fillings depending on the type of Yukkuri or the setting by the Artist.
With many stories skipping on the "living" part and treating Yukkuri as nothing more than food/livestock due to their parallels to manju.