01 -
Heat your oven to 350°F. Mix the brownie mix, eggs, oil, and water according to the box directions for "cakey" brownies. Pour the batter into a greased 13×9-inch baking pan and smooth it out evenly.
02 -
Bake at 350°F for about 20 minutes, until they're almost done but not quite. Check with a toothpick - it should come out with a few moist crumbs (slightly under-baked).
03 -
Take the pan out of the oven and immediately sprinkle the graham cracker pieces evenly over the warm brownies. Next, scatter the mini marshmallows on top, then finish by arranging the Hershey's chocolate squares throughout.
04 -
Return the pan to the oven for another 5 minutes, just until the marshmallows puff up and turn lightly golden, and the chocolate pieces get all melty and delicious.
05 -
For warm gooey brownies, spray a knife with cooking spray and cut while still warm (keep spraying between cuts to prevent sticking). Or for neater squares, let the brownies cool completely before cutting into 20 pieces (5×4 grid).