Preheat oven to 175°C. Line a square baking pan with parchment paper and lightly grease it.
In a bowl, mix graham cracker crumbs, melted butter, and sugar until evenly combined and crumbly.
Press the mixture firmly into the bottom of the pan to form an even crust layer.
Bake the crust for 8 to 10 minutes, then remove and let it cool slightly.
Prepare the brownie batter according to package instructions, mixing until smooth.
Pour the brownie batter over the crust and spread evenly using a spatula.
Bake for 25 to 30 minutes or until the center is just set with moist crumbs.
Remove from the oven and immediately sprinkle marshmallows and chocolate chips over the top.
Return to the oven and bake for another 5 to 7 minutes until marshmallows are soft and lightly golden.
Allow brownies to cool completely in the pan before lifting out and slicing into squares.