Preheat the oven to 350°F (175°C). Line an 8x8 inch (20x20 cm) or 9x9 inch (23x23 cm) baking dish with parchment paper and set aside.
In a large mixing bowl, using a handheld or stand mixer fitted with a whisk or paddle attachment, mix butter and sugar on medium speed just until combined for about 1 minute. Add the egg and mix just until combined. Mix in flour, graham cracker crumbs, baking powder, and salt just until incorporated.
Divide cookie dough in half. Transfer one half of the dough to the prepared pan and press into the bottom of the pan evenly. Lift out the dough layer with the parchment paper and set aside. This will be your top cookie dough layer.
Line the baking pan again with parchment paper and press the remaining cookie dough into the bottom of the pan evenly. Spread the peanut butter evenly on top if used. Then place the chocolate bars in a single layer on top and spread the marshmallow fluff evenly on top. Place the top cookie dough layer on top and carefully press into the pan to even.
Bake for about 24-26 minutes when using a 9x9 inch baking pan or for 26-28 minutes when using an 8x8 inch baking pan or until golden brown. Let the bars cool to room temperature before cutting. Store leftovers in an airtight container at room temperature up to 3 days.