Onsen-manju reffers to a manju (a bun stuffed with azuki-bean paste) sold in towns and resorts of hot springs.
It is supposedly named after that hot spring water is used for its dough and the steam from hot spring is also used for steaming the buns. However, there are only few hot springs which have both sodium bicarbonate suited for making soft and full dough, and steam of high temperature required for steaming, therefore many of the manju called Onsen-manju are normal manju just as a souvenirs. In other word, it became called 'Onsen-manju' since it was manju sold in towns of hot springs; and Onsen-manju became a standard Japanese confectionery sold in souvenir shops or served in a Japanese-style hotel of towns of hot springs.
In all parts of Japan, Onsen-manju refers to mainly steamed manju and its color is white or brown, however it is not rare to find unique Onsen-manju which are made for meeting various tastes.
As for the birthplace of Onsen-manju, someone says it was Gunma Prefecture, and someone says it was Ikaho-onsen hot springs, however there is no evidence that similar manju had not existed before that. In the Meiji period, people of a town of hot spring were trying to make a new confectionery to be sold as a specialty, and the people gained some hints from Usukawa-manju (steamed yeast bun with filling with thin skin). It is easy to imagine that the success case among them triggered the spread to hot springs of all parts of the country.
In the past, in Ikaho-onsen hot springs which has been called the birthplace of Onsen-manju, the water of the spring's source and the mineral deposits of the hot springs (resembling flowers) were used for adding the unique color which reminded of spring's source to the manju. However the result was not desirable, therefore it is said that confectioners used unrefined sugar, which was becoming available at that period, as a substitute for water and mineral deposits. Confectioners of other towns of hot springs also tried blending water of spring's source, and someone tried making it by using high temperature steam of hot springs.
As for the production of manju using the steam from hot springs, there remains a record of the Edo period; according to it, at 'Furo no yu,' one of spring's sources of Atami-onsen hot springs, such steam was used for making manju. However it was recorded that the manju made at that time was for a staple diet not as a souvenir.
Yunohana manju (Ikaho-onsen hot springs in Gunma Prefecture)
Chichiya Onsen-manju (Kusatsu-onsen hot springs in Gunma Prefecture)