This is a Japanese sweet with a red bean paste filling. It is similar to Dorayaki, which consists of two pancakes sandwiched together with the filling in between. One can also use other sweet filling i.e. vanilla custard or savoury filling i.e. curry, potatoes. etc.
It is said that imagawayaki has its roots in Kanda, Tokyo. 200 years ago, in Kanda's Imagawabashi neighborhood there was a sweet shop that sold red bean paste filled buns which over time became known as imagawayaki. These simple flour buns filled with anko soon became popular as the perfect mid-afternoon snack.