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 cold brew and 1 ½ tablespoon marshmallow syrup. Stir.
Top with marshmallow cold foam. Drizzle with mocha sauce and sprinkle with crushed graham crackers. Drink up!