Preheat the oven to 350 degrees and line an 8x8 baking dish with aluminum foil, so that it goes up and over the sides -- this will make it much easier to get the bars out of the pan to cut later.
In a large bowl, add the butter and brown sugar. Use a hand mixer (alternatively, you could use a stand mixer with the paddle attachment) to cream the butter and sugar together on medium speed, for about 2 minutes. Add the vanilla and egg, beating until combined.
Add the flour, graham cracker crumbs and baking powder to the bowl, and with the mixer on low, gently combine.
Press ⅔ of the cookie dough into the bottom of your prepared pan. Spread the marshmallow cream evenly over the cookie dough and sprinkle with the chopped Reese’s Peanut Butter Cups. Break the remaining cookie dough up over the top and mold into a flat layer -- it won’t fully cover and that’s okay.
Bake for 25 minutes, or until the top is golden brown. Remove from the oven and cool completely on a wire rack before removing from the pan. Cut into squares and enjoy!
Store at room temperature in a sealed container for up to a week. If storing in the refrigerator, I’d recommend microwaving for 15-20 seconds to warm them through and turn the marshmallow cream gooey again.