Shake Shack, Milk Bar churn up 2 new birthday-inspired shakes

Shake Shack's sweet and creamy blended treat lineup is expanding in a new collaboration with New York City-based dessert destination -- Milk Bar.

The two brands teamed up to blend Shake Shack's frozen custards with Milk Bar’s modern desserts to add two new shakes to the menu.

Christina Tosi, Chef and Founder of Milk Bar told "Good Morning America" she happily jumped at the opportunity "to make milkshake dreams come true with our BFFs at Shake Shack."

One is a chocolate birthday cake shake featuring Milk Bar B’Day crumbs, the gourmet version of funfetti cake, and Milk Bar B’Day frosting hand-spun with chocolate cake frozen custard and topped with whipped cream and rainbow sprinkles.

The other is a cornflake chocolate drizzle shake made with Milk Bar's signature Cornflake Crunch, hand-spun with malted vanilla frozen custard and topped with Milk Bar chocolate fudge drizzle and mini marshmallows.

"We settled on our hand-spun shakes as the muse, and then Christina and her team at Milk Bar did what they do so well - crafting fun and delicious takes on some of their most legendary baked goods, but in our frozen custard," Shake Shack culinary director Mark Rosati said. "We had a great time working on this and can’t wait for our fans to stop by, order one or two shakes, and enjoy!"

The shake duo will hit menus on Sept. 3 at Shake Shack locations nationwide and for pickup and delivery on Shake Shack’s website or app. Fans who use the Shack App can snag exclusive early access starting Aug. 31.

To sweeten the deal, Shake Shack announced that it will give away free shakes to lucky fans in honor of their birthdays. Every Friday in September, customers can check Shake Shack’s Instagram Stories where the company use its B’Day Randomizer feature to select a few random dates. If your birthday is selected and you're one of the first 50 followers to slide into Shake Shack’s DMs, the brand will send a discount code to redeem on the app for a free shake.