Preheat the oven to 350°F. Line an 8x8 inch baking dish with parchment paper.
In the bowl of a stand mixer (or in a large bowl with an electric hand mixer), add the butter, granulated sugar and brown sugar. Cream together until well-mixed, 2-3 minutes.
Add the egg and vanilla extract and mix until combined.
Add the gluten-free flour, baking powder and salt. Mix on low speed until totally combined, scraping down the bowl once halfway through.
Stir in the gluten-free graham cracker crumbs.
Press 1/2 of the dough into the bottom of the parchment-lined pan. Lift out the parchment with the dough on it and set it aside – this will be your top cookie layer.
Line the pan with parchment again and press the remaining dough into the bottom of the pan. Top with the chocolate bars, then spread the marshmallow creme over the top of the chocolate. Flip the top cookie layer on top.
Bake for 25-30 minutes, or until golden brown. For best results, let cool completely before serving. Enjoy!