Line a 9-by-13-inch cake pan with parchment, allowing it to hang over the sides. Coat with cooking spray.
Whisk together the milk and eggs in a medium bowl. Fold in the brownie mix until you have a smooth mixture. Stir in the butter.
Pour 2 cups of the batter into the prepared pan and spread to even out the surface. Drop spoonfuls of the marshmallow cream on top of the batter and use a wooden skewer or knife to create a swirling pattern. Pour the remaining brownie batter over the top. Arrange the segments of chocolate bar on top. Next, sprinkle over half of the graham crackers.
Bake for 30 minutes, then remove from the oven and sprinkle over the mini marshmallows. Bake until the marshmallows start to brown a little, an additional 10 minutes, then remove from the oven and allow to cool for about 5 minutes. Drizzle over the salted caramel. Sprinkle the remaining graham crackers, chopped chocolate bar, mini chocolate candies and sprinkles over the caramel sauce.
Using the parchment, remove the whole brownie bar from the pan. Slice into squares and serve.