Preheat the oven to 350 F. In a medium bowl, whisk together flour, cocoa powder, salt, and baking soda. In a large bowl, combine butter with sugar until fluffy.
Add in egg and vanilla extract and mix until it is smooth. Add in the flour mixture, and mix until it is fully combined. Fold in the crushed graham crackers, chocolate chips, and mini marshmallow bits.
Press the cookie dough evenly into a greased 8-inch square baking dish and bake for 20-25 minutes.