Bun with croquettes. コロッケパン。A type of sozai-pan, buns with savory fillings ("sozai" is the term for the side dishes eaten with rice).
Korokke (croquesttes) were introduced from the West in around 1900. They are a popular form of Yoshoku (Japanese-style western food).
A hot dog bun is sliced open and filled with one or more croquettes. The coquettes can be of various types. Most common are potato croquettes with some Japanese Worcester sauce. Sometimes shredded cabbage is added as well. Other types used are for example fish or meat croquettes.