My last monthly Japanese breakfast column for The Japan Times was on onigiri. The highlight of my research was this lovely gem, Sendagi Risaku. All of the other shops were part of a chain, but this was an independent shop that, for me, is worth having on your radar when you visit the Yanaka area.
The Japan Times column for more details:
Bunkyo-ku, Sendagi 2-31-6 文京区千駄木2-31-6
opens 8 a.m.
closest station: Sendagi