While the brownie is cooking place graham crackers in a large Ziploc bag and crush them into small pieces using a heavy glass or rolling pin. Yes, you can buy graham cracker crumbles, either way is fine.
Once you have fine crumbs, place the graham cracker crumbs in a bowl and add sugar and 2 tbsp butter, melted works well. Mix together with a fork until mixture is moist and holds together well. You can add additional butter if needed.
Total brownie cooking time should be around 30-35 minutes, depending on the oven. When the brownie is almost done, around 20 minutes take it out of the oven and place graham cracker crumb mixture over the brownie and pat it down. (Use oven mitts to handle pan, it will be hot.)