The Shack-cago Dog ($4.25) starts with a snappy Vienna Beef frank that's cooked on a roller-top griddle, then cradled in a steamed poppyseed bun. But what distinguishes this Chicago specialty is the onion, cucumber, tomato, pickle, and sport pepper, plus relish, celery salt, and mustard (but never ketchup), that suggest it's been "dragged through the garden," the saying goes. Some folks wrinkle their noses at so many toppings on such a skinny dog; I like it. (Simple is good, too.)
I'm less fond of the fries ($2.75). They're prepared from frozen crinkle-cuts, it's been reported, so they're consistent from batch to batch but also, somehow, lacking. The "potato pixie dust" that remains when the fries are gone held no magic for me. Salt, mostly.
The burgers ($4.75) are excellent, especially when you consider that they're grilled and dressed to order; the burger below, caught in apparent midflight from Madison Square Park, was medium rare, with lettuce, tomato, pickles, and onion.
Shake Shack's frozen custard is rich and smooth, without any of the suspect chemical notes that often infiltrate soft-serve ice cream (a related but distinct product with less butterfat and no egg yolk). In addition to vanilla and chocolate, you'll find seven other flavors every month, one for each day of the week. Summer peach, sour cherry, salted caramel, mild but unmistakeable sweet corn (below; one scoop; $3.25), and a headier balsamic fig (below that) are only a sampling.
366 Columbus Ave. (at 77th St.)
Also inside Madison Square Park, near 23rd St. and Madison Ave.