tayaki is basically a waffley sandwich that can be stuffed with savoury or sweet surpises. they're usually (and i believe traditionally) shaped like carp.
savoury fillings:
- tuna (from a can... gross)
and sweet fillings:
- custard
- red bean paste
- glutinous rice balls
this one is from a small stand in the koenji district of tokyo and stuffed with red bean paste (azuki paste).
i don't know much more about these things because i can't read very much japanese.