Datemaki is a rolled sweet omelet mixed with surimi(fish paste).Usually It is made from egg, surimi, soy sauce, mirin ( or sake ), sugar, salt, and so on.
If you cook it at home, you can mix hanpen as surimi to egg.
Baked omelet is rolled with bamboo mat while it is still warm.
Datemaki is a typical new year's food in Japan.
