When Gillie Houston first tried the trendy, low-cal ice cream Halo Top, the experience was “life-changing.” An entire pint has as little as 240 calories — compared to about 1,000 in your average container of Ben & Jerry’s — and, some say, tastes as good as a non-diet dessert. “I think there’s some kind of witchcraft or sorcery involved in the making of it,” says Houston, 24, a writer who lives in Prospect-Lefferts Gardens and has a food-centric Instagram account, @gilliehouston, with more than 101,000 followers.