Preheat oven to 350F (175C)—line parchment paper in an 8x8-inch baking pan.
Prepare brownie batter: in a bowl, add melted butter, water, and egg. Mix, then add the brownie mix.
Break pieces of 1 1/2 graham crackers and sprinkle them on the brownie batter. (spread the pieces all around using a toothpick or chopsticks, graham pieces are all up to your preference).
Bake for about 23-25 minutes or until a toothpick inserted into the center comes clean.
Once done, remove the brownies from the oven, and switch the oven to HI BROIL.
Top the baked brownies with marshmallows (space them out as much as possible). Broil for 2-3 minutes or until the marshmallows are charred and melted.
Use a toothpick or marshmallow to spread out the marshmallow (this can get messy, but who cares, it'll taste so good).
Add the rest of the graham cracker pieces on top of the marshmallows.
Let cool completely before cutting into squares.