In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
In a large bowl using a hand mixer (or stand mixer), beat the butter, brown sugar and granulated sugar together until light and fluffy, about 1 minute. Mix in the egg and vanilla until combined.
With the mixer on low, slowly add in the flour mixture and mix just until incorporated, about 20 seconds. Add in the graham cracker crumbs and stir just until combined, about 10 seconds (either with the mixer or by hand).
Press half of the dough into the bottom of the pan. Lift out the parchment with the dough on it and set it aside—this will be your top cookie layer.¹
Line the baking pan with parchment paper again and press the remaining dough into the bottom of the pan. Layer with the chocolate bars, marshmallow creme, and top cookie layer.²
Bake at 350° F for 30 minutes or until golden brown on top. Let cool completely for best serving results.
Refer to the recipe video for visual demonstration of creating graham cracker layers.
I suggest flipping the top cookie layer upside down then peeling off the parchment paper. This will create a smoother top layer (refer to the video).
Store in an airtight container at room temperature for up to a week.