1Heat oven to 350°F (325°F for dark or nonstick pan). Grease or spray bottom only of 13x9-inch pan with baking spray with flour.
2In small bowl, toss chocolate chips with 1 tablespoon of the cake mix. In large bowl, beat remaining cake mix, 1/4 cup dry chai latte mix, the water, oil and eggs with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Stir in coated chocolate chips. Pour into pan.
3Bake as directed on box for 13x9-inch pan. Cool completely, about 1 hour.
4In medium bowl, mix powdered sugar and butter until smooth; set aside. In small microwavable bowl, microwave milk on High 10 to 15 seconds or until very warm. Stir in 1 tablespoon dry chai latte mix until dissolved; stir into powdered sugar mixture until smooth and spreadable. Spread over cake. Just before serving, sprinkle with cinnamon. Store loosely covered.