Mark is cheap, delicious, and comfortable. The menu is tiny: sliders, fries, chili, pie, milkshakes, beer. Hidden away under a staircase on St. Mark’s between 2nd and 3rd Aves., it’s a long, skinny place that looks like a bar but feels like a café, with an open front wall, little ottomans to sit on, and no crowd at all when I was there with my friends Meredith and Jordan. Our waitress described these as “like White Castle but better”, which is an understatement. They do taste a bit like White Castle’s, but they’re at least twice the size, meaning you don’t need a sack of 10 to get full, and…