1-Preheat your oven to 350°F (177°C). Line an 8-inch or 9-inch square pan with parchment paper, leaving some overhang on the sides. This will make it much easier to remove the bars later.

2-In a mixing bowl, cream together the unsalted butter and packed brown sugar using an electric mixer on medium-high speed. Continue beating for about 2 minutes until the mixture is light and fluffy.

3-Add the room temperature egg and vanilla extract to the butter-sugar mixture. Beat until well combined. Don’t worry if the mixture looks a bit curdled at this point this is normal and will smooth out when we add the dry ingredients.

4-In a separate bowl, whisk together the all-purpose flour, graham cracker crumbs, baking powder, and salt. Gradually mix this dry ingredient blend into the wet ingredients until just combined. Be careful not to overmix, as this can result in tough cookie bars.

5-Press about two-thirds of the cookie dough into the bottom of your prepared pan. Use your hands or the back of a spoon to create an even layer. Don’t worry if it seems thin it will puff up during baking.

6-Spread the marshmallow creme evenly over the cookie dough layer. An offset spatula works well for this step, but the back of a spoon can also do the job effectively.

7-Sprinkle the semi-sweet chocolate chips evenly over the marshmallow creme layer.

8-Take the remaining cookie dough and flatten small pieces between your hands. Arrange these pieces over the chocolate chips. It’s okay if the dough doesn’t fully cover the marshmallow and chocolate layers this creates the beautiful marbled effect we’re looking for.

9-Bake in the preheated oven for 25-30 minutes, or until the top is lightly golden brown. The center should still seem slightly soft to the touch.

10-Allow the bars to cool completely on a wire rack. This is an important step, as cutting them while warm will cause them to fall apart. Once cooled, use the parchment paper overhang to lift the bars out of the pan and cut them into squares.