Preheat your oven to 350°F (180°C) and line a 9×9-inch baking pan with parchment paper or aluminum foil, making sure the lining extends over the sides for easy removal. Set the pan aside.

In a large mixing bowl, combine the melted butter and brown sugar, whisking until smooth. Add the egg and vanilla extract, whisk again until fully incorporated. Next, add the flour, graham cracker crumbs, baking soda, and salt. Stir until the mixture is just combined and a soft dough forms.

Press half of the dough evenly into the bottom of the prepared pan. Drop about half of the cookie butter in 9 small dollops over the dough. Use a knife or skewer to gently swirl the cookie butter into the batter.

Place the chocolate bars on top of the swirled layer, breaking them as needed to create an even layer that reaches all edges and corners. Spread the marshmallow fluff evenly over the chocolate layer. Dollop the remaining cookie butter over the fluff and lightly swirl it in for a marbled effect.

Take the remaining dough and flatten it between your hands into pieces or discs. Gently place them over the top, patching them together to fully cover the marshmallow layer. It’s okay if some marshmallow peeks through.

Bake for 25 to 30 minutes, or until the top is golden and set. Let the bars cool completely in the pan before lifting them out—cutting too early will result in a gooey (but delicious) mess!