What adds to the anticipation of getting pastry chef Zac Young's Flex Donuts is watching him hold a large pastry bag and inject small, round doughnuts with filling. Young, who is executive pastry chef at Flex Mussels and a former contestant of Top Chef Just Desserts, is selling his doughnuts ($1.50) for two weeks in the takeout section of Zocalo in the Lowel Level Dining Concourse at Grand Central Terminal. Young offers five flavors: salted cajeta (caramelized goat's milk), tequila-lime, raspberry, spiced chocolate, and cinnamon sugar. More