Melt butter and stir in sugar and graham crackers crumbs. Press into the bottom of a 9x13 pan lined with parchment paper. Make sure the parchment paper overlaps so you can easily remove the bars from the pan. (Note: This recipe is doubled to make a thick crust. If you want a thinner graham cracker crust then cut crust ingredients in half.)
Freeze for 5-10 minutes until set.
In a large bowl, stir together the brownie mix, eggs, oil and water until well mixed. Pour onto graham cracker crust.
Bake according to package directions.
Remove brownies from oven and immediately layer the top with marshmallows and return to oven for 2-3 minutes. For the last minutes, turn on broiler to brown the marshmallow tops.
Allow to cool and carefully move from pan onto a serving tray or cutting board.