1. Prepping Your Baking Pan

Preheat your oven to 350°F (177°C). Line an 8-inch or 9-inch square baking pan with parchment paper, ensuring enough overhang on the sides to easily remove the bars after cooling. Set the pan aside while you prepare the dough.

2. Making the Cookie Dough

In a large bowl, using a handheld or stand mixer fitted with a paddle attachment, cream the softened butter and brown sugar together on medium-high speed until light and fluffy (about 2 minutes). Add the egg and vanilla extract, then beat until fully combined. Don’t worry if the mixture looks slightly curdled; it will come together when the dry ingredients are added.

3. Combining Dry Ingredients

In a separate medium bowl, whisk together the flour, graham cracker crumbs, baking powder, and salt. Slowly add this dry mixture to the wet ingredients, beating on medium speed until just combined.

4. Assembling the Cookie Bars

Press or spread about two-thirds of the graham cookie dough into the prepared pan to form a thin, even layer. Since the dough is sticky, you may need to use your hands or a spatula. Spread the marshmallow creme evenly over the dough. It may be a bit tricky since the creme is sticky, but do your best to create an even layer. Sprinkle the chocolate chips evenly over the marshmallow layer.

Mold the remaining cookie dough into flat pieces and place them on top of the chocolate chips. Don’t worry if some marshmallow or chocolate chips are exposed; this will add to the rustic appearance of the bars.

5. Baking the Cookie Bars

Bake in the preheated oven for 25-30 minutes or until the top is lightly golden brown. Remove from the oven and place the pan on a wire rack to cool completely.

6. Cooling and Cutting

Once the bars have cooled completely, lift them out of the pan using the parchment paper overhang. Cut into squares and enjoy!