I think it is more correct to steam steamed buns with a steamer. Because using a plate instead of a steamer will cause uneven heating, the steamed buns will not be cooked, but the steamer will dissipate heat, and the steamed buns will be heated evenly, cooked quickly inside and outside, and the shape will be good. So don't use a plate instead of a steamer to steam steamed buns.

picture of steamed buns in a steamer.