Preheat your oven to 350F and spray a 9x13-inch baking dish with non-stick cooking spray.
Combine graham cracker crumbs, ½ cup granulated sugar, and 6 tablespoons melted butter in a medium mixing bowl. Mix well, then pour the mixture into the prepared baking dish and press evenly to form the crust layer. Set aside.
Whisk together the flour, cocoa powder, and baking powder in another medium mixing bowl; set aside.
In a large mixing bowl, use an electric mixer on medium speed to cream together butter and sugar until light and fluffy.
Add milk and vanilla extract to the bowl, and mix well.
Add eggs one at a time, mixing between each addition.
Gradually add the dry mixture to the wet mixture until just combined. Fold in chopped peanut butter cups.
Pour the brownie batter over the graham cracker layer in the baking dish.
Bake the brownies at 350F for 35-40 minutes, until a toothpick inserted into the center comes out clean.
Remove the brownies from the oven and set the oven to broil.
Sprinkle peanut butter chips over the hot brownies, then top with marshmallows.
Place the pan of brownies in the oven under the broiler and broil for 1-2 minutes until the marshmallows are toasted. Be careful not to burn them! Keep a close eye on the brownies and remove them from the oven when the marshmallows are golden brown on top.
Let the brownies cool for at least 10 minutes before slicing them into 12 bars.
Enjoy your delicious peanut butter cup s'mores brownies!