Preheat the oven to 350F. Spray an 8" square baking dish with nonstick cooking spray. Set aside.
In a 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.
Add the eggs and vanilla extract, and beat until the color becomes a pale yellow - about 2 minutes on medium-high speed.
Add in the flour and cocoa powder and mix at low speed just until no large lumps remain. Be sure not to overmix!
Scrap down the sides of the bowl using a silicone spatula to ensure the batter is fully combined. Transfer the batter to a prepared baking dish. Top with mini marshmallows and flakey salt.
Bake in a pre-heated oven for 25-30 minutes, or until the center is no longer jiggle when gently shaken.
While the brownies are still warm, sprinkle over the crushed graham crackers. Allow the brownies to cool for at least 30 minutes before slicing.