Do yourself a favor and take Chef Tien's cooking class.
The entire experience is great, from sampling local cuisine, hitting the market for your ingredients, and of course learning to cook traditional Vietnamese food. Chef Tien and his staff are all great people - it was very nice getting to know them all a little bit.
The food that you wind up making is top notch as well. All my life, I was used to eating LA-style Pho. Needless to say, once I had one sip of Chef Tien's broth - my view on good pho changed. This is by far the best pho I've ever had.
Lots of places advertise cooking classes in Hanoi, but they seem kind of like tourist traps. With Chef Tien, you learn how to cook - but you also get to hang out with great people and experience a little bit of Hanoi culture.