Preheat oven to 350º Fahrenheit.
In a medium mixing bowl, beat butter and sugars together.
Add egg and vanilla and mix well.
Beat in flour, baking powder, and salt.
With a mixing spoon, stir in graham cracker crumbs.
Line an 8x8 baking dish with parchment paper.
Press half of the crust mixture into the baking dish.
When you have it flat, gently grab the parchment paper and lift the top layer out of the baking dish. Set it aside.
To create the bottom layer, line the baking dish with parchment paper again and press the remaining dough into the bottom.
Spread the peanut butter over the top (I like to microwave the peanut butter for 10-15 seconds, so it is easier to spread).
Evenly place the Reese’s Peanut Butter Cups on the bottom layer.
Sprinkle marshmallows on top.
Cover with the remaining layer of crust.
Bake for 30 minutes or until top is golden brown.