Preheat the oven to 350 degrees. Add 12 Graham Cracker Sheets and 2 tbsp Sugar to a food processor and blend until uniform crumbs form. Add 6 tbsp Salted Butter (melted) and continue to mix until the mixture resembles wet sand.
Line an 8x8 baking pan (ceramic or metal - do not use glass) with parchment paper. Add the graham cracker crumbs and press firmly into the bottom of the pan in an even layer. Bake for 12 minutes then set aside to cool.
While the crust is cooling, make the brownie base - add 1/2 cup Salted Butter (melted), 3 tbsp Vegetable Oil, 1 cup Sugar, 1/3 cup Brown Sugar, 2 Eggs, 1 Egg Yolk , and 1 tsp Vanilla Extract to a large bowl. Whisk together for 2-3 minutes, until the sugar is dissolved and the mixture is smooth and lightened in color. Then add 3 tbsp Sour Cream and 3/4 cup Dutch Process Cocoa Powder and mix until smooth.
Add in 2/3 cup All Purpose Flour and 1/2 tsp Kosher Salt and fold together until combined.
Pour half of the brownie batter on top of the crust, then add 4 graham cracker sheets on top and sprinkle with chocolate chips. Add the remaining batter on top, ensuring that the filling is completely covered.
Bake for 28-32 minutes, until the edges are set and the center is slightly jiggly. Allow the brownies to cool at room temperature for about 30 minutes, then place into the refrigerator for 1 1/2 hours to set.
Before serving, make the marshmallow topping (you can also skip this and use store bought marshmallow fluff). Bring 2 inches of water to a boil in a small saucepan. Once boiling, reduce heat to low and simmer.
Add 4 Egg Whites and 1 cup Sugar to a clean heat safe bowl. Place the bowl on top of the saucepan and whisk until the sugar has dissolved, about 5-6 minutes. (It's dissolved when you no longer see sugar granules - if you touch it and rub the liquid between your fingers, it should feel smooth. If it's sandy, continue to cook) The temperature of the mixture should be 165 degrees F if using a candy thermometer.
Remove from the heat add 1/2 tsp Cream of Tartar and either use a hand mixer or pour into the bowl of a stand mixer fitted with the whisk attachment. Mix on medium speed for about 1 minute, then increase to high speed and continue to mix until stiff peaks form, about 5-7 minutes. Fold in 1 tsp Vanilla Extract and add on top of the brownies.
Optional: torch the top of the fluff for toasted marshmallow flavor!