For the Graham Cracker Crust

1. Preheat your oven to 350 degrees Fahrenheit. Line an 8×8 pan with parchment paper and spray with cooking spray.
2. In a medium bowl, use a fork to combine fine graham cracker crumbs, melted butter, and granulated sugar together. Mix for about 2 minutes to ensure the butter has fully distributed into the crumbs.
3. Pour the crumbs into the prepared pan. Bake the crust for about 5 minutes until lightly golden brown.

For the Brownies + Topping

1. In a small saucepan over medium heat, stir chopped butter and chocolate chips until smooth. Stir constantly to prevent the mixture from burning.
2. In a large bowl, whisk granulated sugar, light brown sugar, and eggs together until mixture turns light pale in color. This should be about 2 minutes of mixing. Add vanilla extract and stir to combine.
3. Add flour, cocoa powder, and salt to the batter, and whisk until the dry ingredients are just combined. Do not mix past when you see all of the dry ingredients disappear into the batter, as overmixing will lead to dry brownies.
4. Pour half of the brownie batter on top of the baked graham cracker crust. Place Kit Kats on top of the brownie batter, making sure to cover as much of the batter as possible. Pour remaining brownie batter on top of the Kit Kats and spread with a knife.
5. Bake brownies for 30-40 minutes or until a toothpick inserted into the center comes out with moist batter. This ensures that your brownies come out fudgy and chewy.
6. Remove the pan from the oven before topping the cooked brownies with mini marshmallows. Place the pan back in the oven for 2 minutes to soften the marshmallows.
7. Place chopped Kit Kat bars and crushed graham crackers into the marshmallows.
8. Let the brownies cool for at least an hour. Do not slice before the brownies have cooled. Store brownies, covered, at room temperature for up to 4 days.