A steamed Japanese cake
In onsen areas with shops, you often see "Manjuu", a steamed Japanese cake with a doughy outside, and sweet red beans on the inside.
At one time, they would use actual onsen water for the dough, and the steam from the onsen to cook the manjuu, but these days, just selling them at onsen is enough for them to be called "onsen manjuu".
The most common story around onsen manjuu is that they came from Ikaho in Gunma prefecture, and had the characteristic color of the onsen there.
Manjuu sold at onsen areas that are actually steamed from the hot springs are soft and delicious. Other onsen areas have their own unique styles and flavors of manjuu as well.