室町砂場 日本橋本店 Lunch Earlier This Week @ 'The Sandpit' in Nihonbashi, Tokyo

Sunaba was the first Soba shop to serve tempura soba, apparently. Before them, the buckwheat noodles and the tempura were served separately. The Sunaba 'tsuyu' sauce is unusually smoky and strong. Not everyone's cup of tea, but I love it personally.
砂場は天婦羅そばの発明所です。このつゆは凄く濃いです。関東のあじでしょう。

http://r.gnavi.co.jp/g212300/

Dashimaki is pretty damn good too.