Preheat the oven to 350 degrees Fahrenheit. Line an 8x8-inch pan with parchment paper and spray it with cooking spray.
In a large bowl, or mixer, whisk the butter, sugar, and brown sugar until fluffy. Whisk in the egg and vanilla until combined.
With the mixer on low, stir in the flour, baking powder, salt, and graham cracker crumbs until combined.
Spread 2/3 of the dough into the bottom of the prepared baking pan. Press the peanut butter cups gently into the dough. Spread the marshmallow fluff on top.
Press the remaining dough onto a non-stick surface into an 8x8-inch square, lay it on top of the marshmallow fluff, and press lightly.
Bake for 25-30 minutes or until the edges brown. Allow them to cool for at least 2 hours to set. Remove from pan and cut into squares.