This unassuming street cart belies what is surely the best 40 baht hamburger in the city. For another few baht, you can top it off with cheese and bacon. Add a side of onion rings or mashed potatoes and you still won't break 100b. They also use actual farang condiments, like French's mustard and sour mayonnaise. The beef patty is cooked on the griddle along with eggs, onions and steaks*, just like your dirty hometown diner, and they're open till midnight. Which is pretty much exactly when you'd want to go.
Tell them not to put the bun on the griddle - it just gets all greasy.
* "Steak" is best avoided at pretty much any roadside stall. It's usually a crap piece of meat cooked well-done with a side of soggy french fries. Mark does a better job than most, but the burger is far superior.