The Birthday Cake Shake is ice cream and milk blended with yellow cake and sweet buttercream frosting, finished with whipped topping and rainbow sprinkles. It normally goes for $3.49.
Why a birthday sing-along? The burger chain is celebrating its 80th birthday and expects over 20,000 customers to sing "Happy Birthday" simultaneously at its 529 US locations.
In case, you're looking for something more to commemorate the experience, they're also giving out free Special 80th Birthday versions of their classic paper hats all summer long.