CHRISTINA TOSI is the pastry chef-owner of Momofuku Milk Bar, with locations in New York and Toronto, and the pastry chef of all of the four Momofuku restaurants in New York. Milk Bar has cookie mixes sold through Williams Sonoma, a wedding cake department that bakes for clients such as John Legend and Chrissy Tiegen, and a line of "fashionably wholesome" cookies with Karlie Kloss and FEED called Karlie's Kookies.