Preheat oven to 350 degrees F. Line an 8x8 pan with parchment paper and spray with cooking spray.
Line bottom of pan with 4½-5 graham crackers, breaking squares off to fit to form a base. Set aside.
Prepare brownie mix according to box instructions, adding the vanilla and espresso powder with the other ingredients. Mix until just combined, then fold in the caramel sauce, chocolate chunks, ¾ cup marshmallows, and remaining crumbled graham crackers.
Pour batter into prepared pan and smooth out. Bake for 15 minutes.
Open the oven and carefully scatter the remaining marshmallows evenly on top of the setting brownie batter. Continue to bake for 25-30 more minutes, until a toothpick inserted in the center comes out clean and the sides are beginning to pull away from the pan..
Cool completely before drizzling with more caramel sauce. Serve and enjoy!