Make the cold foam. Add the heavy cream, marshmallow syrup, and whole milk to a small glass or mason jar and froth with a milk frother until foamy. Set aside.
Add ice to a tall glass, then pour in the chai concentrate, milk of choice, and marshmallow syrup (if using). Stir.
Top with marshmallow cold foam. Drizzle with mocha sauce and sprinkle with crushed graham crackers. Drink up!
Notes
A Note On Marshmallow Syrup: Starbucks uses an unsweetened chai base and adds marshmallow syrup to it. Most chai concentrates (including my homemade version) are already sweetened so there's no real reason to add it unless you really want to. If you're using an unsweetened chai concentrate, add 2 tablespoon marshmallow syrup.