Via Serious Eats.
The new app allows you to view Shake Shack's monthly custard flavors, menus, as well as the line at the Madison Square Park location (updated every six seconds).
The Shake Shack also has new munchkin-like doughnuts (from Doughnut Plant) with hot apple cider (from Minard Farms) for $3.75. Take a look at The Feed.
Original photo can be found here.