Preheat your oven to 350F. Spray an 8” square baking dish with nonstick cooking spray. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar until combined— about 2 minutes on medium-high speed.
Next, add the eggs and vanilla extract and beat until the color becomes a pale yellow— about 2 more minutes on medium-high speed.
Add in the flour and cocoa powder and mix on low speed just until no large lumps remain. Make sure not to over mix!
Scrape down the sides of the bowl using a silicone spatula to ensure the batter is fully combined.
Transfer the batter to the prepared baking dish. Top with mini marshmallows and flakey salt.
Bake in a pre-heated oven for 25-30 minutes, or until the center no longer jiggles when gently shaken. (Important: These are very fudgy brownies. A toothpick inserted into the center will not come out clean).
While the brownies are still warm (and the marshmallows are still gooey), sprinkle over the crushed graham crackers.