Preheat the oven to 350 degrees F. Spray and line a 8x8 inch square pan.
Place the butter and chocolate chips into a microwave safe bowl and melt. Mix together to incorporate. Set aside.
In the bowl of a stand mixer with a whisk attachment, whip the eggs, sugar and vanilla to ribbon stage, or until pale and thick. Slowly add in the melted butter and chocolate and mix to incorporate.
In a separate bowl, sift together the cocoa powder, flour, salt, and baking powder. Slowly add into the wet mixture, a 1/3 of the dry at a time, and fold in by hand with a rubber spatula. Fold in the mini marshmallows, chocolate chips and crushed graham crackers.
Spread the brownie batter in a 8x8 inch square pan and bake for 40-45 minutes or until set.
Top with some more marshmallows and use a torch or broiler to toast the marshmallows.