Preheat oven to 350°F (180°C). Line a 9x13 inch baking dish with parchment paper or aluminum foil, and lightly grease with cooking spray.
In a large bowl, mix together the graham cracker crumbs, sugar, and melted butter until well combined.
Press the mixture evenly into the prepared baking dish, using a flat bottomed glass or your fingers.
Bake the graham cracker crust for 10 minutes, then remove from oven and let it cool for 10 minutes.
In a medium saucepan, melt together the sweetened condensed milk, chocolate chips, and vanilla extract over low heat, stirring constantly until smooth.
Pour the melted chocolate mixture evenly over the cooled graham cracker crust.
Sprinkle the mini marshmallows evenly over the chocolate mixture.
Return the baking dish to the oven and bake for an additional 10-12 minutes, or until the marshmallows are lightly golden brown.
Remove from oven and let it cool completely to room temperature.
Once cooled, cut the chocolate and marshmallow squares into desired size and serve. Enjoy!