The Hanoi banh mi is a different creature from the Saigon version. The baguette is narrower and crunchier, the fillings tend toward pate, cold cuts, and pickled vegetables rather than the wider range of proteins you find in the south, and the whole thing is assembled with less ceremony and more speed. Banh Mi 25, on Hang Ca Street near the Old Quarter, is among the most popular spots in the city for this — a small stall that has been written about extensively without losing its essential character as a fast, cheap, made-to-order sandwich counter.
The standard order is a baguette split lengthways, smeared with butter and pate, layered with sliced cold cuts (cha lua, or Vietnamese bologna), pickled daikon and carrot, fresh coriander, and sliced chili. It costs the equivalent of a few dollars and is made in under two minutes. Go in the morning or at lunch; the bread is best fresh from the supplier and the later-day bread can be less crisp. Take it to eat while walking — this is not a sit-down food.