In a large bowl, cream butter and sugar together until light and fluffy.
In a medium bowl, whisk flour, baking powder, and cocoa.
Add the flour mixture to the butter mixture and using an electric mixer, mix ingredients until well combined.
Next, add milk and vanilla. Mix well.
Add in eggs, one at a time, mixing well after each addition.
Stir chopped peanut butter cups into the batter and then pour batter mixture onto the graham cracker crust and spread evenly.
Place in the oven and bake for 22-25 minutes or until the center is cooked through.
Remove from oven and turn broiler to low.
Evenly spread marshmallows over the brownies. Sprinkle chocolate chips and peanut butter chips on top of the marshmallows and place back in the oven.
Broil just until the marshmallows start to brown and the chips start to melt. Be careful as they burn very quickly.
Remove from oven and let cool for 5-10 minute before serving.