Here's a way to sweeten your commute: Drop by Zac Young's Flex Donuts, which just opened in Grand Central Terminal. The celebrity chef's two-week pop-up shop sells doughnuts for $1.50 each. Patrons can choose from exotic fillings like spiced chocolate, raspberry, cinnamon sugar, salted cajeta and a daily special. What makes it even tastier? Young, the executive pastry chef at Flex Mussels and a frequent guest on Top Chef Just Desserts, will be serving the goods himself. Tue Jan 25--Fri Jan 28, Mon Jan 31--Fri Feb 4 noon--6pm. Flex Donuts is located in Zcalo in the Grand Central Dining Concourse.