If you're looking for "authentic" Northern Thai food, Laab Gai Baan Hong (ลาบไก่บ้านโฮ่ง) is about as real as it gets. They serve plenty of chicken and pork dishes (their name actually refers to "local chicken", i.e. the monsters you see running around instead of factory-farmed), but a good portion of the menu is frog, ant eggs and cow parts you've probably never even thought of eating. Particularly interesting is the udder soup (something like ต้มแซ่บนม), which is made with tamarind, lemon grass, galangal and slices of cow nipple that look exactly like you think they would. They also make a similar soup with cow placenta, which raises an interesting question - if no animal was harmed and the product was going to go to waste anyway, is placenta vegetarian?
Note that there is absolutely zero English here and it's hardly conducive to pointing. Bring a Thai-speaker.