Preheat oven to 350F. Spray a 9x13-inch baking dish with non-stick cooking spray; set aside.

In a medium mixing bowl, combine the graham cracker crumbs, ½ cup granulated sugar, and 6 tablespoons melted butter. Pour the mixture into the greased baking dish and press down evenly to form the crust layer; set aside.

In a medium mixing bowl, whisk together the flour, cocoa powder, and baking powder; set aside.

In a large mixing bowl, cream the butter and sugar together with an electric mixer on medium speed until light and fluffy.

Add the milk and vanilla extract and mix well.

Add the eggs one at a time, mixing between each addition.

Add the dry mixture into the wet and mix just until combined. Fold in the chopped peanut butter cups.

Pour the brownie batter into the pan over the graham cracker layer.

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 the peanut butter chips over the pan of hot brownies, then top with the marshmallows.

Place the pan of brownies in the oven under the broiler and broil for 1-2 minutes until the marshmallows are toasted. Note: do not walk away! The marshmallows will burn very quickly. Stay close and keep an eye on them, removing the pan from the oven the moment the marshmallows are golden brown on top.

Cool the brownies for at least 10 minutes, then slice into 12 bars.