These buns were made using water roux (better known as 'tangzhong') which, if I am not wrong, was originated from Japan. It was popularly raved by many bloggers. Breads made by 'tangzhong' were known to be soft and keep well for a longer period as compared to breads made by other methods. Basically, it is similar to direct method of making bread but requires an additional step of cooking part of the bread flour with water prior to the mixing of the ingredients. This method allows better hydration and thus the flour absorbs more water resulting in a softer texture.