FOR SEVERAL years now, ultracold liquid nitrogen (a chilly minus 321 degrees Fahrenheit, or 77 degrees Kelvin) has been a fad component in super-sophisticated kitchens. As Bee Wilson writes in her delightful history of cooking, Consider the Fork, former Microsoft exec Nathan Myhrvold’s perfect burger is slow-cooked to the proper pink, then plopped in liquid nitrogen (so the middle will stay rosy), then deep-fried for a dark crust. But now Robyn Sue Fisher has brought the surprising culinary delights of liquid nitrogen back down to earth and into the streets to make what many think is the best ice cream ever. She’s patented a machine, which she calls Brrr, that keeps the ingredients smooth as they freeze. (This is the trick, because the idea of using liquid nitrogen to create iced desserts has been around since the late 19th century.) The process is so quick that each batch is made to order, and there are no emulsifiers or preservatives. Fisher started out in 2009 in the parks of San Francisco with the machine in a little red wagon. Now her Smitten Ice Cream is concocted at a stand made from a recycled shipping container, and with rave reviews in tech and foodie publications alike, she looks ready to expand.